What do the British call a television?
The British native languages – especially Celtic.
Among the British languages we have Teledu in Welsh and in Breton the endearing term Skinwel, although despite Britannic it isn’t spoken in the UK. I couldn’t find the Cornish term, but another commentator has it. Nor Manx, although presumably they have them. Irish Gaelic is Teilifís, Scots Gaelic is telebhisean, and the Lowland Scots article in Wikipedia says “Televeesion” although I am not such if that’s official usage, and as far as the Old English word they use, I think we can be pretty sure that’s an anachronism. As Abe Lincoln famously said, “don’t believe everything you read on the internet”. In that vein the Pictish term for TV is VOD as this enables you to “pict” what you like, when you like.
British dialects of English
If you are thinking about British versions of English and the regional or slang terms, I can’t think of any regionalisms. “Telly” is an informal way of talking about television as a service or the actual set, and further slang words for the set exist such as “the box” or “the gogglebox”. The term “tube”, hwever, was not widely used in British English slang and is more of an Americanism which I am not sure many of us would have understood prior to YouTube popularising it.
Community languages
Immigrant communities in the UK exist and the Poles have telewizja, while the words in Indian languages are mainly recognisable as something sounding like the original Standard English word (ie Greek roots put together in such a way that Oedipus could have forgiven his father) but written in their own alphabets and there are quite a few of them.
You can probably get by in the British Isles just using “television”, this weird partly Greek partly Latin word which was put together in the UK by an inventor whose command of physics was clearly many metres per second better than his command of philology.