Chopper Trading is very nice, especially in Chicago. They get season tickets to the Bears, Bulls, Cubs, etc. I know a trader there and I've been in their office on a tour and it's like some edgy tech company's hipster headquarters. They recruit college graduates though. But it doesn't hurt to submit a resume. They have locations in NYC, Chicago, Washington DC, San Francisco, and London.
Tradelink has an office in London I think. They trade futures I believe.
Ronin Capital has offices in London. They are the only prop firm I know of that offers benefits like health and life insurance.
HTG Capital has london offices, but they only seek out algo traders.
That's all I know of that has physical office locations in London. All are pretty competitive. They aren't Jane Street or SMB Capital level, but they aren't scams either.
You can use Google to find their websites. You can find emails and phone numbers.