Berlin blows London away from a cost perspective (YC is ~rich, so the only relevant cost is hotel/etc for attendees who are non-local), and land-travel for a lot of people, but London probably has more local founders than Berlin, especially who know of and would be interested in YC/SS.
UK is not in the Schengen, that's true, but IMO compared to all the hassle of airport security, it's a joke, at least for EU nationals (if you're not from the EU, you'll have to pass the border coming to Berlin as well).
(Someone please correct me if this is wrong.)
Is there a sensible price comparison website that compares costs of cities for things like eating out; drinks; renting apartments; hotel rooms; public transport; etc etc.
Seeing as it's a 1 day event I don't think this is too big of a problem. If cost is an important factor in your decision to come (it would be for me) I'd suggest getting an early flight to London, attending the event, and flying home.
Also, London is not really a tech center anyways. Poor choice.
A lot of tech companies have made London their base, including some prominent YC companies. London is so big that it's many things to many people so if you think it's 'not really a tech center', please elaborate why.
And while London isn't a tech center on the level of US cities, it still is one.