What are culture taboos in the UK?
I can think of a few.
As previously mentioned, it is highly taboo to ask about someone's income. It isn't really done to ask what someone does for a living in the social circles I live in - it will no doubt arise naturally at some point, and your curiousity will be satisfied at that point - but it shouldn't be the first question asked at a social gathering (the most appropriate question would be 'so how do you know the host'?)
It's not done to comment negatively or favourably on someone's education or schooling. The class system here is so ingrained we all take this for granted and have subconscious or conscious views on this. If you're not British, we would take a dim view (i.e. think you are being extremely rude) if you dissed our education or schooling.
It's also very taboo to comment on someone's weight, which is common in Asian countries. I am slightly over average in weight and found Chinese colleague comments on my weight highly offensive. Frankly, it's none of your business. Even commenting positively if someone has lost weight who might have been heavier than average before is pretty unpleasant, they might have a health issue which they don't want to share with you (as there is no close personal relationship) which has been the reason for losing weight. My husband had sarcoidosis for a number of years and dropped a couple of stone from his average weight, but did not want to share this with the wider world and was very upset by some well meaning but overly personal questioning from acquaintances. Impolite and overly-personal questions did not make his experience any easier.
Child-bearing or childfree status - another area which crosses into the health arena. It is the height of rudeness to ask *why* someone doesn't have children. They might have been struggling with fertility issues for years, or have made their own personal choice not to have children. It's not your call to enquire why this is the case and in many circumstances is both personal and painful if it isn't by choice.
It's also highly rude to comment on the size or location of someone's accommodation, as it's seen (indirectly) as a comment in a negative way of their social position, particularly in London. Londoners of many years' standing may be able to negotiate this to a degree, but most people shouldn't make any mention of the fact that you find your co-worker/friend's neighbourhood unsavoury or poor.
All these are different to taboos elsewhere in the Anglosphere or the rest of Europe as far as I understand it, so I think they are peculiarly British.
An interesting one centers on the use of ties. That is the (mostly male) strangling cloth you wind around your neck. Not the social ones, though they play heavily into this particular taboo.
Ties in the UK, at least in certain circles, are markers of social belonging, of tribal identity. Patterns and colors can signify schooling background and college. They can inform about military and social participation.
Misusing these is a cultural taboo. After all, by wearing a Christ College Tie Pattern, you claim membership in that college. To mis-wear ties, amounts to lying about ones social status.
This is taboo.
Living in London is quite a different experience from living in a quiter town, nevertheless I'll give you a major cultural taboo that I've observed as a Londoner;
Interacting with people that you don't need to.
In other words, make no eye contact with people on the tube, if someone needs help ignore them, when you see something clearly wrong or what appears to you as 'immoral' just act as if nothing is happening.
One morning while I was using the underground (the Tube) there was a normal (whatever normal means to you) lady who was just slumping over the stairs. People walked past her as if nothing was happening. I just found it so bizarre as to how people could just ignore someone who so obviously needed help. And that's not the only time I've observed it.
You might say that the example I gave doesn't display a taboo and that it's more to do with society and the way society works etc but to me cultural taboos are all to do with not breaking rules that a culture has created- an aspect of conformity. What are British people best at? Conforming without question.
If you look at the list of cultural taboos listed by most locals, and in particular the residents of Thailand and India (do not spit betel nut juice) they are things most tourists would never do or have even heard of.
In the UK tourists should not criticize the Royal Family.
From statements on this site Americans and even some Brits question the need for the institution, let alone the personality of individual members.
This is the rare taboo that many Americans may consider breaking.
What makes it a taboo is the extreme politeness under normal circumstances of the locals and that for foreigners to question the need for a Royal Family is really provocative.