Nigel Farage will launch an audacious attempt to steal Labour's clothes by unveiling his manifesto in Ed Miliband's seat of Doncaster and promising answers on the cost-of-living crisis and the NHS.

In a speech in central London to mark his victory in the European elections, Farage said his party had not yet reached its peak, and reeled off a raft of seaside and market towns where Ukip hopes to win parliamentary seats next year.

"If any of you think you've seen the high watermark of Ukip, you ain't seen nothing yet," he said. "Our small party isn't so small any more."

Farage listed as potential targets Great Grimsby and Boston in Lincolnshire, Folkestone and Thanet in Kent, Portsmouth, Eastleigh, Plymouth, Aylesbury and parts of Cambridgeshire in the south, as well as Rotherham in the north.

In a particular warning to Miliband, the MEP said he had chosen Doncaster as the perfect place to launch Ukip's general election manifesto in September, having disowned all of the party's policies from 2010.

The Ukip leader said protecting the NHS and tackling the cost of living were two key aims, in a move calculated to win votes off Labour. Although polls show the clear majority of Ukip's votes come from former Conservatives, Farage has repeatedly insisted that his party is eating into Miliband's vote.

"We have already been doing substantial work on the NHS, on defence, on education, on public spending and other areas, and we will unveil our outline manifesto for the next general election, and we will do it in a town called Doncaster," Farage said.

"It is a town in which Ed Miliband is the MP, it's a town in which yesterday we topped the polls, and we will have an honest conversation with the British public about the cost-of-living crisis and about how we can make life better and more affordable for ordinary families in this country. Policy will happen in Doncaster in September."

Ukip topped the polls in the European elections and gained more than 160 council seats in the local elections. In Doncaster, Ukip topped the polls in the European election and came a fairly close second to Labour in the seven local council wards of Miliband's Doncaster North constituency – winning 7,888 votes compared with Labour's 8,948 votes.

Farage's hope of winning seats in future elections are likely to have been boosted by doubling its MEPs in Brussels, as they will be eligible for millions of pounds in funding from the EU over the course of a parliament. In total, the 24 MEPs could claim a maximum of around £44m in salaries, allowances and staffing costs over the next five years if their attendance is exemplary, according to calculations based on figures from OpenEurope.

They will be allowed, and perhaps even expected, to donate some of their £78,000-a-year wages to Ukip for campaigning. And while they are not meant to use expenditure allowances for party business, the extra money for staffing and offices is likely to help the MEPs to professionalise their political operation.

Farage said he was preparing a serious challenge to the mainstream parties at the general election, but the next major landmark would be the Newark byelection. Winning such a safe Tory seat would be a bit like "climbing Everest", but David Cameron would feel the pressure on his leadership if Ukip could pull off such a spectacular win, he said.

Farage said Clegg was already in "the most trouble", but reserved a personal warning for Miliband as well by saying he would launch a new manifesto in Doncaster in September.

The Ukip leader acknowledged the need to run a more professional operation, saying the party would have to confront the fact it had made mistakes. Over the last few weeks of Ukip's campaign, several candidates have been at the centre of controversies about racist and homophobic remarks, despite Farage's promise last summer to weed out "Walter Mittys".

While attacking the media for putting the party under more intense scrutiny than its rivals, he said: "We did not succeed in vetting those people as well as we should have done." However, he said the new crop of 24 MEPs, who flanked him on stage, were of an "infinitely" higher quality than the ones who were elected in 2009, a third of whom had either defected or been sacked over the course of a European parliament.

Labour sources dismissed the threat of Ukip in Doncaster, saying Farage was welcome to try to stand there but that voters would not be impressed by the Ukip leader's boasts about keeping the flame of Thatcherism alive.

On Tuesday Miliband will talk about the changing political landscape when he makes a speech in Thurrock, a marginal Tory-held seat where Labour lost the council to no overall control last Thursday. Following the European results, the Labour leader said he was confident the party was "in a position where we can win the general election".