Firstly, the way the election works (outside Northern Ireland, where they use STV) is you just vote for a party. Each party puts up a list of candidates, you're in London so we have 8 places to fill, but you just vote for a party and the places are divided between them in proportion to the vote. So last time in London, the result was Labour 4, Conservatives 2, UKIP 1, Green 1. And the actual MEPs were the top 4 from Labour's list, the top 2 from the Tory list and so on.
I know this is totally unlike any other British election but the EU insists we must have proportional representation voting, and I suppose the government and Parliament went for PR by regional closed lists because at least choosing how to vote is easy - just pick a party.
Obviously you don't want the Brexit Party or UKIP. The Liberal Democrats, the Greens, and Change UK (which used to be called The Independent Group) want Remain. Labour and the Conservatives are divided but Labour mostly like Remain - in the recent talks, they insisted on a Customs union, which effectively amounts to "no Brexit". I doubt any other party on the ballot paper is likely to get anyone elected. You will also find 11 independent candidates so it will be a long paper.
Edit - I went to vote and it WAS a long paper! You always fold up the paper so nobody sees how you voted when you put it in the ballot box, but this one NEEDED it just to get it in the slot!