Boris Johnson has said he would be willing to increase public borrowing to pay for infrastructure projects as he was pressured over how a government he led could invest heavily in new schemes and public services while also cutting taxes.

Johnson, the overwhelming favourite to replace Theresa May as prime minister in just over three weeks, said that while there was fiscal “headroom” to spend more, if needed he would use low interest rates to borrow for new infrastructure.

Speaking to Sky’s Sophy Ridge on Sunday, Johnson also talked about wanting to raise the national living wage, but seemed unable to say what level it was currently set at.

On Brexit, the former foreign secretary dismissed the unanimous view of EU leaders that the UK’s departure deal could not be renegotiated before 31 October, saying this was a negotiating ruse, and “you would expect them to say that kind of thing”.

Profile Boris Johnson's Tory leadership campaign Show Personal style A late-night altercation between Tory leadership favourite, Boris Johnson, and his partner, Carrie Symonds initially changed the dynamics of Johnson's campaign. He had been either invisible or deliberately sober to the point of dullness, when his usual primary draw to Tory members is a self-created sense of optimism and fun. Much is also made of his supposed broad appeal to the electorate, evidenced by two terms as London mayor. His bizarre claim to make model cardboard buses has raised eyebrows. In most political contests, Johnson’s character – he has lost more than one job for lying, and has a complex and opaque personal life – would be a big issue, but among the Tory faithful he seemingly receives a free pass. It remains to be seen what impact that late-night police visit will have on his chances. Brexit He has promised to push for a new deal while insisting the UK will leave the EU come what may on 31 October, even if it involves no deal. While his hard Brexit supporters are adamant this is a cast-iron guarantee of leaving on that date, elsewhere Johnson has been somewhat less definitive. Asked about the date in a BBC TV debate, Johnson said only that it was 'eminently feasible', although he then went on to tell TalkRadio that the 31 October deadline was 'do or die'. Taxation His main pledge has been to raise the threshold for the 40% higher tax rate from £50,000 to £80,000, at a cost of almost £10bn a year, which would help about 3 million higher earners, a demographic with a fairly sizeable crossover into Tory members. Johnson’s camp insist it would be part of a wider – and so far unknown – package of tax changes. Public spending He has said relatively little, beyond promising a fairly small increase in schools funding, as well as talking about the need to roll out fast broadband across the country. Johnson has generally hinted he would loosen the purse strings, but given his prior fondness for big-ticket projects – London’s cancelled garden bridge, the mooted 'Boris island' airport – perhaps expect more of a focus on infrastructure projects than services. Climate and environment This is unlikely to be a big issue for Conservative party members, and Johnson has not said much on this beyond confirming his general support for the new government target of cutting greenhouse gas emissions to a net zero by 2050. Foreign policy Also unlikely to be a big issue among Tory members, beyond vague platitudes on 'global Britain', it could be a weak spot for Johnson given his poor performance as foreign secretary. He was seen as something of a joke by diplomats – both UK and foreign – and is likely to face more questioning over his gaffe about the jailed British-Iranian woman Nazanin Zaghari-Ratcliffe Peter Walker Political correspondent Photograph: Isabel Infantes/AFP

Johnson was asked how a government of his could spend the promised £4.6bn a year extra on schools, boost transport in the north, roll out UK-wide ultra-fast broadband, have 20,000 extra police, and cut taxes.

Arguing that some of his tax cuts would create extra revenue and stimulate growth, Johnson said: “What is certainly true is at the moment there is cash available. There’s headroom of about £22bn to £25bn at the moment.”

Asked if he would increase borrowing, he said: “If it’s borrowing to finance great infrastructure projects, and there’s the opportunity to borrow at low rates, to do things for the long-term benefit of the country, then we should do them.”

Johnson added: “I’m prepared to borrow to finance certain great objectives, but overall we will keep fiscal responsibility, and keep going with the general trajectory of ensuring that this country pays its way and lives within its means. But you can do that with some sensible tax cuts that will stimulate growth, stimulate enterprise, get more enterprise in.”

Johnson said he would want to increase the national living wage – the new official name for the minimum wage – but declined to say by how much: “I certainly believe in lifting the living wage, but I’m not going to set a figure now.”

Asked by Ridge how much the living wage currently was – it is £8.21 an hour – Johnson said “around £10 or so”. He explained that he had instead meant the London living wage, currently £10.55 an hour, which is a higher, non-compulsory figure intended to reflect real living costs.

On Brexit, Johnson dismissed the repeated comments by EU figures that his stated plan to renegotiate the withdrawal agreement could not happen, saying this view would rapidly change once he became prime minister.

“Possibly it’s the case that they would say that at this particular stage in the negotiations. You would expect them to say that kind of thing,” he said.

“I don’t think they are going to want a disorderly Brexit of any kind. It’s not in their interest. There’s no reason why they should want the sudden imposition of tariffs and quotas.”

Johnson repeatedly refused, again, to accept personal responsibility for comments he made about Nazanin Zaghari-Ratcliffe, the British-Iranian women jailed in Tehran, and whether they, as her family believe, had worsened her plight.

“Of course I feel very bad about all the consular cases in Iran, particularly Nazanin,” he said, insisting that Iran was solely responsible and it was wrong to “allow whatever I may have said or done to cloud the issue”.

He similarly dismissed criticism of his comments from the past about black and gay people, and Muslim women who wear full-face veils.

Asked by Ridge whether he would “just say anything to get a laugh”, Johnson replied: “No. I think if you look at each and every one of those columns or articles you’ll find that the quotations have been wrenched out of context, in many cases made to mean the opposite of what was intended.”

Quizzed on whether he took responsibility for anything, Johnson said he would so for Brexit.

“I played a part in that campaign and I’m very proud of what we did but I take personal responsibility now for what is happening to our country, for the drift and the dither, and the indecision, and the failure to be sufficiently robust in the negotiations which we’ve seen so far,” he said.

“What I want to add now is my own ability, I think, to lead us out of this mess, to get Brexit done by 31 October, and then to do for the whole country what we did in London which is to bring it together.”