How expensive is London?

My girlfriend found plane tickets for €50 for the end of January, including a two-night stay in London, and asked if we wanted to go. If there are cheaper options, she'd be happy to go with me for just one night. But that would be even less worthwhile. I'm wondering how expensive the buses and trains are there? And what about food? Airbnb is relatively cheap, but if you end up spending €400-€500, that's a lot for just two or three days 😂

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