Thanks for the answers to my questions and the additional information you've provided about her.
No worries about the picture but it may be helpful in trying to figure out what's going on with her skin
I absolutely understand the quandary that you're in about her: you want to help but you don't want to put your other cats at risk either.
Without a physical exam, I do have several thoughts about her, especially since she's itchy. Cats can develop ringworm or fungal infections where they'll lose fur but they typically aren't as itchy as she sounds. So while I couldn't rule it out entirely, this seems much less likely...which is a good thing in terms of contagious conditions.
1. Fleas. That's good that she doesn't appear to have any fleas but I'll include them on the list to be complete since Flea Bite Dermatitis is such a commonly seen problem in cats.
You may or may not see fleas since they can hop off and on a cat's body and they can be ingested from all the grooming. Cats can become allergic to the flea saliva and literally one or two fleas can drive them crazy. And, the areas of their bodies that they tend to lick a lot are the spine in front of the tail and the stomach; sometimes the backs of the legs, too, and around the neck. So, I would want to rule this problem out just to be safe by use of a good topical flea product. I'm not certain what you might have available otc in the UK but Advantage or Cheristin are good topical products.
2. Food allergies which is usually to food that a cat has been eating for a while--it's not new food as you might suspect.
Changing to a hypoallergenic diet may be helpful such as grain-free, or Natural Ba***** *****mited Ingredients, Nature's Variety Instincts line, Evo Duck or Venison, Nature's Variety Frozen Raw Medallions (I recommend that they be zapped in the microwave for 10-15 seconds on each side).
Of course, we don't know who else may be feeding her so 100% control of her diet may not be achievable but it's likely something fairly easy for you to do.
3. Inhalant allergies to such things as pollens, dust mites, mold, grasses, trees, etc. These cats can lick just about anywhere on their bodies with this problem. They may or may not respond to antihistamines (see below) but they almost always respond to steroids which can help with the diagnosis.
4. There are other external parasites such as Demodectic Mange or Notoedric mange which are pretty itchy conditions. They tend to be fairly uncommon here in the States although I'll include them to be complete.
These are treatable conditions which is a good thing; usually whatever you use for fleas will kill these mites as well....Frontline (Fipronil) or Revolution ( Selamectin) are two examples.
As to possible over the counter treatments to help reduce the itch level, antihistamines can help in some cases. However, since sedation can be a reaction to these drugs, I'd be concerned that she might be at risk for predators if her response time is blunted. But, I'll provide a few examples on the off chance that you'd consider them:
Diphenhydramine at a dose of 1/2 of a 25 mg tablet given twice daily or
Chlorpheniramine at a dose of 2-4 mg twice a day or
Loratadine at a dose of 2.5-5 mg/cat or
Cetirizine 5 mg/ once or twice a day
It is important to ensure that the formulations used contain ONLY the antihistamine and are not combination products (e.g. Claritin-D contains pseudoephedrine, which could cause very significant adverse effects in a cat).
Sedation is a common side effect with these kinds of drugs.
I don't typically suggest or advise that anything topical be used on a cat's body since they'll lick it off which might cause more harm; however, Vetericyn Spray is safe to use on cats even if they ingest it. I honestly don't know if it will be of benefit but it won't be toxic which is the important thing.
I'll be more than happy to view the picture tomorrow and I can revise my response, if necessary. I have a pretty good idea of what her skin looks like but it may be helpful to know what's in my mind's eye is actually what her skin looks like.
In the meantime, I hope this helps. Deb