Sounds about right, the London Living Wage (a suggested but not mandatory minimum wage for living in London) works out at just under £400 per week, so maybe £5k to live on, the remaining £5k for other expenses, etc.
Bear in mind it's not £2.5k/month for living expenses: it's £2.5k per month for all expenses: including building the product, etc etc.
That is living an fairly decent life. You could scrape on £1k a month if you really wanted to.
This is what I do, rent and bills is around £800 a month, and can live quite well on another £300 to £400, and that includes going out.
I spent 5 years living like that at my first couple companies. These days, however, my life is no longer organised in a way where that's viable. So 2-2.5k it is. If I was in a real pinch, I could temporarily get to 1.5k, but I feel like 4 months is about the right period of time for this anyway.
That's about me, post-tax for the past two years I think. Would go a lot further if I were single, but we can afford to rent a decent-sized 2-bedroom flat in a nice area without having to pinch pennies.
It's fairly common to pay at least £1k/month for a one bedroom flat in central London.