Why do some British (English) people add a pronoun to the end of a sentence?

It looks like right dislocation to me, just with utterances without an explicit subject. (It’s not always a pronoun and the concept itself isn’t specific to England.)

Here’s a paper with a similar example:

Situation: A has told B about how he slammed his finger in a car door.
B: Agonizing, that. Car doors are always a problem.

Growing up in Yorkshire, I was accustomed to emphasising the main thought about something by stating it and then referring to the something by using this or that.

Looking at a three-legged dog the main thought to be expressed is that it is unusual. Then comes the indicative reference to the thing that is unusual: “Unusual dog, that.” or “Unusual, that dog.”

Wikipedia has relevant accounts that put my childhood experience in wider context:

English is a so called subject-prominent language, in which the subject is usually prominent. Another type of language is topic-prominent:

Although English is a subject-prominent language, it does have ways of producing topic prominent sentences. These include terms like “as for” and “regarding”, as well as few structural rearrangements such as the passive voice, fronting and clefting.

Fronting refers to those odd sentences you come across sometimes where the topic has been moved to the front. They feel awkward because English is a subject-prominent language but their subject doesn’t come first.

A couple of examples with the subject highlighted in yellow, and the topic highlighted in green:

Chocolate, I like.

Americans, he can’t stand.

Wikipedia

This movement so as to emphasise the topic may be left or right (as with my Yorkshire dog example).

There are two types of dislocation: right dislocation, in which the constituent is postponed (as in the above example), or a left dislocation, in which it is advanced. Right dislocation often occurs with a clarifying afterthought: They went to the store is a coherent sentence, but Mary and Peter is added afterward to clarify exactly who they are. By contrast, left dislocation is like clefting: it can be used to emphasize or define a topic. For example, the sentence This little girl, the dog bit her has the same meaning as The dog bit this little girl but it emphasizes that the little girl (and not the dog) is the topic of interest. One might expect the next sentence to be The little girl needs to see a doctor, rather than The dog needs to be leashed. This type of dislocation is a feature of topic-prominent languages.

Wikipedia