As Mirrajay says - you need to go through the PETS process first, but I can give you a general idea on airfare as I recently moved four cats over. The PETS process probably cost me somewhere in the region of $1000 altogether for bloodwork etc, USDA vet signature for all four cats together.
I just realized - I say recently but it's been almost a year since I moved over!! So airfare prices may be slightly out of date.
I used Virgin Atlantic, direct flight from Washington Dulles to Heathrow and it was approx $1200 for airfare for all four cats together (they calculate this by volume, not weight, by the way). I then had to pay another lump sum + some amount for each extra pet (in my case, three extra as I had four total cats) at the Animal Reception Center in Heathrow - but I can't remember the exact amount. Somewhere in the region of £400-600 altogether probably.
Should give you a general idea...