Plaid Cymru leader Leanne Wood questions whether UK parties will deliver on promises if voters say no to independence

The Plaid Cymru leader Leanne Wood has warned Scottish voters they cannot trust "dithering" Westminster politicians to honour any promises to increase Scotland's powers if they vote no to independence.

Wood told the Guardian that the UK parties had already obstructed and watered down proposals for greater Welsh devolution, including new powers to control income tax in Wales proposed by the independent Silk commission set up by the UK government and backed by Labour.

The Westminster government and Welsh Labour had cherry-picked some powers suggested by Paul Silk's reviews, and then reneging on others such as the power for Wales to set variable rates of income tax, Wood said.

"The UK parties can offer you a lot and promise you jam tomorrow but in terms of delivery, we have to question whether or not they will deliver the goods," Wood said, before a series of meetings with Alex Salmond and senior Scottish government ministers in Edinburgh.

"The danger is that unionist politicians are more interested in preserving the union rather than delivering Welsh self-government in the way that Silk has proposed."

Danny Alexander, the chief secretary to the Treasury, told Wood earlier this year the UK government had blocked Silk's call for Wales to have the power to cut some taxes below the UK rate because it would damage the wider economy.

Wood's warnings echo complaints by the Scottish government and yes campaign that Labour, the Tories and Lib Dems will fail to honour their promises during the referendum campaign to greatly increase Holyrood's powers after a no vote.

Scottish Labour is offering greater but not complete control over income tax rates and housing benefit, withdrawing an earlier proposal to hand over full income tax powers to Scotland. The Tories' devolution commission under Lord Strathclyde is also expected to offer greater tax powers later this month.

They and the Lib Dems are adamant that Holyrood will get extra tax and financial powers after a no vote, in addition to existing powers to vary Scottish income tax rates and introduce new house sales taxes by 2016. But that process is likely to take some years to complete.

But despite backing the SNP stance on extra powers, Wood confirmed that Plaid would be campaigning for the overhaul of the Treasury's Barnett Formula which gives Scotland £1,300 more per head in central government spending, if there was a no vote.

That would likely put Plaid in conflict with the SNP, which has warned voters that Scotland's funding is at risk following a no vote, by splitting the nationalist group at Westminster, Wood confirmed.

Plaid had been campaigning to reform Barnett for 15 years, Wood said, but added that the two parties had a long-standing understanding that they would stand up for their own country's interests.

"We want to see reform of Barnett, yes," Wood said. "We have an arrangement with the SNP where we work we them in two institutions [the UK and European parliaments] but we're very clear that the SNP will put the best interests of Scotland first and Plaid Cymru will put the best interests of Wales first.

"We are within our rights to fight for the interests of both those countries and if those interests are different, so be it."

She said Plaid agreed with complaints from other Welsh parties that the Barnett formula short-changes Wales; estimates suggest that it gets between £300m and £450m less per year than it deserves from the Treasury.

Meanwhile, English businesses, council leaders and some Westminster MPS want Scotland's share of UK spending cut – a mood confirmed on Wednesday by a British Chambers of Commerce survey which showed nearly two thirds of businesses in England, Wales and Northern Ireland want the formula reformed.

Speaking before her meeting with the first minister, Wood said Plaid's over-riding objective was to see the SNP win the independence referendum. She anticipated that Plaid Cymru activists and members of the Welsh assembly would travel to Scotland to help the yes campaign during the referendum campaign, using up their summer holidays.

Wood said she did not rule out coming to campaign in Scotland herself. "There are members of Plaid Cymru who are very keen to be involved and will be helping people to do that," she said. "In the summer, I know of people who will be coming up here, especially in August when they've got more time off work."

She said a yes vote in the Scottish referendum would reinvigorate the devolution agenda in Wales and would likely accelerate the transfer of powers from London to Cardiff.

Wood said an independent Scotland would be a counterbalance to London's influence and the greater Scottish prosperity post-independence would increase Wales' potential to export to Scotland – an argument the UK parties and many economists dispute.

Scotland's emphasis on defending public services, universal benefits and the NHS would be a model for Welsh politicians fighting the UK government's attacks on public services.

"The opportunity to rebalance things socially and the emphasis on public services [in Scotland] is the opposite to the austerity drive coming from London," she said. "The opportunity that Scottish independence gives the rest of us throughout the UK to opt for an alternative to austerity."