Havent done it either way but expat forums have threads on the subject. Try
www.andalucia.com/forums. Animals .
First suggestion is to have a look on the ferries websites that link uk to Spain or France for their requirements for travel.
Second point is you need paperwork , a pet passport per animal, and the entries for vaccinations etc need to have been completed in the correct ink colour.
The next point is that time might not be your friend. If i was going the other way i.e. Spain to uk then rabies injection needs to be uptodate. Then my dog has to be wormed within a narrow window before the ferry crossing. ... a problem if you have a leisurely jorney before ferry or if there are bank holidays. Not impossible but just needs planning.
Pet will also need a microchip as part of the pet passport.
Final thought is does the cat travel well. Each vet or kennels trip for our cat is a real problem with vomitting twice in 30 miles. The only time we didnt have a problem was when we got hold of a tranquilser tablet from our vet. A third was the right dose. Vet wouldnt supply it after that even though it worked well. Your vet could no doubt help for the one journey.
Air carriage. I have no experience but have seen animals caged up at Malaga airport and understand it isnt cheap. Captive audience unlike human passengers. Also not all airlines carry animals. More stuff to research urgently. Might involve indirect flight e.g. London madrid, madrid to island via major carrier.
A few thoughts, hope it is of use.