A British diplomat responsible for promoting the UK as a "proven gateway" to the European Union for investors in the Asia-Pacific region has been awarded the €100,000 Brexit prize for outlining the most compelling blueprint for a withdrawal from the EU.

Iain Mansfield, the director of trade and investment at the UK embassy in Manila, who declared that a British exit from the EU would free the City from "European interference", received his prize from the former chancellor Lord Lawson.

The award of the prize to a serving diplomat who works for UK Trade & Investment, which describes Britain's membership of the EU as one of key reasons for overseas businesses to invest in Britain, is likely to raise eyebrows.

In the first of seven reasons selling Britain as a strong market for investors, the UKTI says: "The UK's open and internationally focused economy is one of the largest and most sophisticated in the world and a proven gateway to the US$17tn EU market."

In his entry for the Brexit prize, promoted by the Thatcherite Institute for Economic Affairs thinktank, Mansfield takes a different view. He says: "Exiting from the EU should be used as an opportunity to embrace openness.

"The outcome would be to accelerate the shifting pattern of UK's exports and total trade away from the EU to the emerging markets, where the majority of the world's growth is located. A more business-friendly regulatory regime and the new security of the City of London from European interference will enhance competitiveness and compensate for the partial loss of access to the European market."

Campaigners for a British exit may be disappointed that Mansfield suggests there may be only a small economic upside to a British withdrawal – a 1.1% increase in GDP, which works out at £1.3bn. He says that in the worst-case scenario Britain could lose more than twice that amount (2.6% of GDP) after a withdrawal.

Mansfield argues that leaving the EU is a political rather than an economic decision, and he is not advocating a particular course. He writes: "It is abundantly clear that the UK can have a positive economic future either inside or outside the EU. Canada, a smaller economy than the UK, prospers alongside its much larger neighbour, the US.

"This paper does not … address the question of whether or not the UK should leave, or advocate for or against such a course of action. What it does do is demonstrate that, in the event of such an exit, there exists a scenario for an open, prosperous and globally engaged UK that is eminently achievable."

Lawson, chairman of the Brexit prize committee, who called last year for Britain to leave the EU, had no idea that the submission was written by a diplomat who promotes Britain as a strong investment destination within the EU.

The committee selected Mansfield from a shortlist of six who were all given four months to write submissions of 10,000 to 20,000 words after they were whittled down from 149 entrants who submitted proposals of no more than 2,000 words.

The rules of the Brexit Prize stated that the winner would be the candidate who sets out the most compelling blueprint for how Britain would negotiate its withdrawal from the EU and how it would forge new trading relations once it had left.

Lawson said: "The British people have been promised a referendum on whether the UK should leave the EU. There are few more important decisions than this. In advance of that decision the issues need to be fully debated, and an essential element of the debate will be an understanding of how the UK might conduct itself if and when we leave.

"Iain Mansfield's prize-winning entry provides an excellent starting point for this important debate, written with the experience of a serving member of the diplomatic corps with a solid background in trade policy."

Mansfield, 30, who is married with a young child, is something of a polymath. He achieved a Cambridge first in natural sciences (physics), writes a regular blog, creates games and has published a novel, Imperial Visions, under the name IG Mansfield.