Tbh, I was hoping some other Brit with actual social skills would drop by with the answer, then I could pretend to have known all along. I think it indicates increasing familiarity, something like
- Tea - you are a person who might want tea
- Cuppa - we are on friendly terms and I consider you my social equal
- Brew - I would trust you with my life and call you wanker to your face
Good shout. There's probably class differences too - there usually are.