The UK can complete the negotiations for a new trading relationship with the EU inside the two-year Brexit timetable since UK and EU trading regulations are already in complete convergence, Sir Tim Barrow, the new UK envoy to the EU, has said.

He also said the EU had already “welcomed the clarity of the UK naming a date for the triggering of article 50”, saying the date was full square inside the government’s previously announced timetable.

Barrow, making his first extended appearance since taking over as UK envoy from Sir Ivan Rogers, said article 50 put the EU under a duty to negotiate not just the terms of the divorce, but also the future UK-EU trading relationship.

Barrow, in effect the UK’s chief day-to-day negotiator over the next few years, was speaking in front of the Commons EU scrutiny committee on the day Downing Street announced that the letter triggering the start of talks would be sent to Brussels on 29 March. He said article 50 required the EU as a matter of law to take into account the future framework of UK-EU relations.

By contrast, many EU politicians have insisted that the talks on the terms of the divorce must be fully complete before discussions can start on a future free trading relationship, a sequencing that would make it much harder for the UK to reach an agreement within the two-year timetable set out in article 50.

Both Barrow and Brexit minister, David Jones, pointed out that the chief EU negotiator, Michel Barnier, said the entire talks could be completed inside 18 months to give time for the necessary democratic authorisation to be agreed. In a bullish performance, Jones said there was nothing the EU could do to stop the UK leaving the EU. “What is absolutely clear is that our departure cannot be stopped because we have the right to leave,” he said.

Offering a similar view of the talks ahead, Barrow said: “A lot of the speculation [about how long it will take] is based on a different sort of negotiation – a free-trade agreement, but between countries which are not in convergence and seeking to find a way to bridge gaps.”

By contrast, UK and EU trading standards would be in complete convergence on the date the UK left the EU, he said. His remarks omit the difficulty the two sides will face in negotiating an agreed future framework to determine the oversight of future trading.

But Barrow insisted: “Our mandate is clear: to get on with it. There is a timetable which everyone has bought into, included in the treaty of two years, so that’s what we’re going to do.” Jones added that the negotiations would “reflect the fact that there’s a strong mutual interest in maintaining good businesses relationships between the UK and the EU”.

But Barrow accepted that the more the final agreement included elements of a new UK-EU trading relationship, as opposed to simply the terms of the divorce, the more likely lawyers would view the deal as a mixed agreement, so giving EU member state national and regional parliaments a right to vote on the deal.

The UK recognises that the more the deal requires wider democratic endorsement, the harder it will be to push the deal through, so putting pressure on the EU and the UK to keep the deal as simple as possible.

Barrow, who also pressed by another Conservative MP, Jacob Rees-Mogg, to say whether the UK would pay “a brass farthing” to the EU, replied that a recent Lords committee report asserting there was no legal need to pay any exit fee to the EU was “very helpful and had been noticed in Brussels”. Detailed talks about the UK’s so-called divorce bill are likely to feature at the start of the talks. Some EU officials claim the UK must pay as much as €60bn (£52bn).

The EU scrutiny committee chairman, the Conservative MP Sir Bill Cash, urged the UK negotiators to point out during the talks that the UK wrote off half of Germany debts after the second world war, and as a result did not owe the Germans anything politically or legally.

Cash said the UK had been “net contributors for many decades” and the “massive contributions” paid to Brussels would offset any divorce fee demanded by officials. He urged the government to remind Europe’s leaders of the 1953 London debt agreement, “where Germany, for all its malfeasance during the second world war” and unprovoked aggression, had half its debt cancelled.

Cash said that given Germany’s dominant role in the EU, it might be worth “tactfully” reminding people “that there is a realistic position here that we really don’t owe anything to the EU, whether it’s legal or political”.

Jones, mapping out the negotiating terrain facing the UK, also said that many European capitals, as well as Barnier, had shown a great deal of understanding about the special status of Ireland, as the only EU country that has a border with the UK. The UK is pressing for the current soft border between Northern Ireland and Ireland to be retained, allowing the free movement of people and goods.

Jones also said there would be a large number of departmental bills required to implement changes from EU policy at the point the UK leaves the EU. A white paper would be published shortly, he said, setting out the plan for the great repeal bill that will incorporate EU law into UK law at the point of UK departure.