The problem with the rental market in London is that there are a lot of people who move to London for jobs/early in their career or students who don't want to commit to buying a place so are happy to rent. Then landlords realise that if they can afford to pay the deposit, they can get some sucker to pay their mortgage (in a lot of places, rent is as much or more than what a mortgage repayment would be with historically low interest rates).
I moved to London 5 years ago and it's insane how much property prices have increased in my area in that time period. Some estate agents were saying 350 p/w for a one bed flat. And that's not even looking at the prices to buy.