What are all the ways the British use the word "lovely"? Especially towards pretty girls?

From watching many period dramas and plays set in England, as I like to do, I've become more acutely aware of the British overloading of the word lovely. In particular, I have two questions:

What are all the contexts in which the word is used? For example, what does lovely mean as an interjection, as opposed to, say, great? Secondly, and more importantly, what are the connotations when lovely is applied to people? For example, is it quite strong to publicly say something like,

She's a lovely woman.

or

Mum, I met a lovely girl at the cinema last night.

Does saying lovely in either one imply a romantic or sexual connotation above mere admiration of the woman or girl?

It varies. Both of your examples might indicate a sexual or romantic interest, or it might not, depending on the proclivities and status of the speaker; alternatively they might be general expressions of excellent in the object. It might or might not refer to excellence of physical appearance, but I think that is not usually the primary meaning.

With inanimate objects (including plants and the like), it most likely refers to excellence in the sensual or aesthetic qualities of the item.

Lovely is as ambiguous as love.

As an interjection lovely can be used pretty much interchangeably with great, but emphasizes the positive emotion which it engenders in the recipient, rather than the greatness of the objective result. That also rules out its use in many situations where the right, proper, or dutiful thing is done (which can make it great) but where negative emotion rules out it being seen as a positive.

That evil murderer was jailed for life today. Great!

not

That evil murderer was jailed for life today. Lovely!

Of course, it can be use ironically in such a context:

I can smell sewage. Lovely!

Used to describe people, lovely sits alongside fine, nice, and so on in an extremely complex category of words.

Lovely can be used perfectly plainly to describe kindness and geniality and won't be mistaken for anything else where more lascivious meaning would be inappropriate. You can, without fear, describe your gran, your boss, or your neighbour's kittens as lovely (or nice).

In situations where the question of romantic interest may be raised in the listener, things get more complicated. It can be used in three ways. First, it can be used as above to mean a perfectly Platonic ἀγάπη, human kindness, warm feelings. Second it can be used to mean intense desire, or erotic pleasure.

In most cultures this would make the word useless, but English being what it is, that gives rise to its most common use which is to inject deliberate ambiguity, often for ironic effect.

When someone who has been dismissing other matches out of hand unexpectedly describes a new match as lovely, they are introducing the second meaning along with the first, so prick up your ears. Whereas when a father has been trying to persuade his daughter to marry Mr Goodbody, and she relpies "He's lovely" expect a "... but ..." to follow.