The bills are indeed often horrific. Can't help feeling that the books are highly cooked against the leaseholders' favour, meaning bills of 10,000s that the leaseholders have to pay. You'd be paying way above what's normal market value for the works whether they be new windows, utilities, upkeep of communal areas, roofs and guttering, green space, carparks etc. The problem is you can't just pay for your eg. own new windows (which is typical for most leaseholders in purpose built developments) but do have to pay up for the figure on which Southwark agree with a contractor. (IMO tendering will likely involve bidding contractors each quoting very high sums, so difficult for you to argue against it.) On the plus side you're likely to be able to pay for works in instalments and pass on such dues to the next owner when you sell if you've not finished paying.
If you love the flat then get on and buy it, but take detailed written and verbal advice from your solicitor during the transaction. It's a good place to live, so good luck! 👍🏻