USD 350 on the flight for the cat (if permitted breed), the UK Health certificate you'd need to check on the DEFRA website for a vet near you and ask them what they would charge.

You'd also need to plan ahead and make sure your cat has a rabies shot and do a antibody test (a month after the rabies shot), as that has to be 3 months old to return to UK. You also cant travel back the same way - as its manifested cargo only into the UK.
(I'm a pet shipper)