Probably £55 - £80k a year, depending on how 'hot' the stack you're on is and how willing you are to work for hedge funds / banks.
I've heard a rumour Halifax might give mortgages to contractors based on a pro-rated day rate...
There are also specialist contractor brokers, and I got a mortgage approval for a decent amount through one of them last year, they offered me way more than I could afford.
HSBC don't understand contractors completely but they have offered me a reasonable amount of lending (I make about half what you do, but then I choose to live elsewhere). HSBC seem to have about the best rates at the moment.