There aren't any catteries in Milton Keynes itself, or if there are I couldn't find any by searching on the net! The closest FAB one is at least an hours drive. There's quite a few in villages on the outskirts though, I went and looked at two last week and have just booked the boys into Ivy Lane cattery in Great Brickhill - 01525 261808 - it's probably about 7 or 8 miles from Fishermead.
It will be their first time in a cattery so I haven't used it yet but I was very impressed when I went to look at it. It's cat only (no dogs), purpose built, very secure, very clean, nice spacious runs with an indoor and outdoor section. The owner was happy for me to turn up and inspect it with no notice (just travelling time from my home). I've booked them in for the summer holiday but will still use a pet sitter for weekend breaks etc as I think it's less upheaval for them.