i questioned this too, above, and the best anyone could come up woith is financial services - which i queirred as to how long it would take for them to replicate the necessary skills to not need us.
but yea looking at a bigger picture, it can increase the scope and depth of the EU market, making it more favourable to import into - knock on effect their growth may necessitate further services being imported from the EU (UK) to allow them to increase infrastructure. its a bit tenous, but still..
also, another more comcrete example, large chains are buying businesses in europe, inc ex soviet states, and become part of the EU may open them up to buyers in the UK looking to expand, especially in the retail/tecnology sector where UK finance can provide investment which isnt necessarily obtyainable in said country.
this fuels the growth of the UK backing firm, providing more jobs (potentially) in the UK, and in other EU countries too. - inc value of company - inc shareholder wealth - benefitting general public holding shares, also investment companies, who can then go on to invest in further startups - which may mean further capital in the UK + further investing, which means more jobs available = favourable to the EU and UK?