Yes, it is legal to own a fox in the UK. The reason is as follows. The fox is specifically “excepted” meaning not subject to the rules regarding the keeping of dangerous wild animals as set out in two pieces of British legislation namely: The Dangerous Wild Animals Act 1976, which should be read in conjunction with the Dangerous Wild Animals Act 1976 (Modification) (No.2) Order 2007. The latter has a schedule which sets out what the British government believes are dangerous wild animals requiring a licence to keep them. The fox is part of the Canidae family but is specifically outside of the rules as are raccoon dogs and domestic dogs but not the dingo.
Conclusion: to restate, it is legal to own a fox in the UK because they are not listed as a dangerous wild animal under the relevant legislation of the country.
Mange
Below is a photo of the vixen I cured of mange and with whom I became friendly.

I would like to make a plea to all people who are considering keeping a fox as a pet. Obviously do all the research you need to do to carry out your responsibilities to a high standard as a fox caregiver. And, read this linked page on a free homeopathic treatment which is guaranteed to cure a fox of mange.
Mange is a terrible disease causing great distress in foxes and it is very common. All you have to do is feed a wild fox and place on the food half a dozen drops of this homeopathic medicine and within about two months the fox will be free of mange. You will be doing a great service and it is a very easy project to carry out.
In fact, is a nice way to part-own a fox by feeding a fox that may frequent your garden and with whom you become familiar. It’s a way becoming friendly with a fox. I’ve done this and I have fed a fox by hand after a few months of becoming acquainted with a vixen. Each time that I fed the fox I put half a dozen drops of this treatment on their food. Very simple, very efficient and very pleasing.