Right off the bat, the first thing that hit me was 'I wonder if there's an infection going on in there?
I've never had a cat have a polyp removed but I did extensive 'reading up' on polyps and their removal when I suspected and the Vet wondered about Stryker of having one. I don't remember all the details now except that these are not simple surgeries and that the removal process sounded almost barbaric to me....basically, the polyp is just pulled out ! It just seems to me that the trauma of that could well produce swelling in the surrounding tissue. Now, given that and adding in what you're seeing, then, if swelling is a natural outcome of the removal, I would expect the Vet to have told you - to allay your fears by providing a logical explanation.
It is not difficult to determine whether/not an infection is at play...a simple temperature-taking and a minor blood test will do it in most cases.
Because this area is so close to the brain, I wouldn't be leaving things any longer.......on the face of it, things are NOT improving and, in fact, you're seeing signs of a worsening, imo.
Because you're in the UK, I want to mention a couple of cautions for you. There are two medications that are given like candy there: one, an injectable antibiotic and, the second, an 'anti-inflammatory'. Each has significant risks for cats and worse, given together, they are most often fatal.
The antibiotic is called "Convenia" - you can read about it here: Convenia: Worth the Risk? by Lisa A. Pierson, DVM :: Long-acting antibiotic for cats that can have serious side effects
The 'anti-inflammatory' comes in each of "Metacam" (oral & injectable) and "Onsior" (tablets). We humans rely heavily on non-steroidal anti-inflammatories (NSAIDs) for pain relief.....cats are really different, though and are unique in their inability to metabolize them...a cat's liver does not have the requisite enzymes to deal with them - so, it's the kidneys that must deal with them 'on the front line'...and, these substances destroy kidney tissue. Some degree of kidney damage is the invariable result.
Naturally occurring swelling from surgery (not the result of infection) will self-resolve without anti-inflammatories, it will 'simply' take a little longer.
Now, if pain is a factor, there are other (opiod) drugs that are very safe for cats...best to use those.
If Timmy were here, I would be having him seen as quickly as possible and reassessed....by a different, competent Vet. If you have the resources for an after-hours examination, I'd recommend it.
Out of curiosity, are you within accessible range of UCD Veterinary Hospital ? Here's their webpage: UCD Veterinary Hospital, UVH
Because it's part of the Veterinary school, there'll be a full range of certified specialists there. It may be a resource that you'll need to insist on being referred to, although I hope not. (I see that people can 'book themselves in', though it seems to be a deliberately difficult admission process that they've set up....quite differential to the Veterinary 'community' if you ask me ! [Quelle surprise !])
I'm really bothered by (what sounds like, to me) the cavalier 'attitude' of the Vet and the immediate conclusion that the cat 'MUST HAVE' sustained a facial injury.....
(This part's opinion only: I find, from reading personal experiences with UK Vets, that many do not feel the need to explain anything to the "owner"....it's an attitude of 'I'm the Vet, you are ONLY the uneducated owner....you have no need to meddle with medical information, just do the aftercare as you are told'.)
Now, I don't know if any of that is involved in your situation or not. I DO know that I wouldn't be leaving this any longer by taking a wait-and-see approach.
Hope something there helps...and, post back about any of it.