You'll want to call the airlines to get quotes.
This page has the options for flying your pets into the UK according to DEFRA's rules. Looks like direct to Manchester, you can leave from Atlanta, Fort Lauderdale, Las Vegas, Newark, New York, and Sanford FL. So if you don't live near any of those places, you may have to book a domestic flight to get your cats there first, or you'd have to fly into London and go get them.
From calling British Airways and Virgin Atlantic several times, I've gotten a different quote each time. (Unfortunately, they seem to be going up steadily.)
The airlines will have requirements as far as what documentation you need (health certificates, etc) to fly, what size crate they'll need to be in (some require more head room than others), and obviously how much it'll cost.