Is only 13% of the UK economy and only 8% of its businesses involved with European Union trade?

CAPX recently published a pro-Brexit opinion piece by John Longworth who headed a pro-Brexit pressure group. He argued that the UK were being held to ransom and were being treated like a “vassal state” of the EU in trade negotiations.

He included a specific, unreferenced statistic that makes a strong and checkable claim:

Another interesting statistic is that only 13% of the UK economy and 8% of businesses are involved with EU trade, the remainder being either purely domestic operators or trading instead with the rest of the world. Any deal needs to be primarily favourable to these firms, rather than the EU and also to our predominant service sector, for which there remains no single market in the EU to speak of. The EU was after all, designed to suit German manufactures and French agriculture.

It is the part above (highlighted by me) that I’m interested in (I left the remaining argument in for context only).

I’m skeptical for more than one reason. It doesn’t feel right even for me as a consumer. More importantly, the quote doesn’t answer the “% of what” question. For example, when it says 8% of businesses, is it simply counting the proportion of all businesses of any size or is it counting a weighted percentage by size? And do the numbers mean anything useful in the context of a complex trading economy?

Are only 13% of the UK economy and 8% of businesses involved with EU trade?