I've not flown a cat back into England but I did bring my dog back into England (well, 2 of them) via the ferry into Dover.
You just go to a vet wherever you are (can't remember if there are requirements about the vet in particular), get the tick and tapeworm treatment done (make sure its of the variety that it needs to be), have the vet fill out the correct section in the passport and stamp it. Make sure its filled out correctly! Date/time/etc. Also make sure its done within the proper timeframe of returning. Check DEFRA's website for all the particulars.
its pretty much a breeze with the passport assuming you've got all the right sections completed.