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"Don't pronounce the Rs in a word except the beginning, if a word ends in an a, say the a all drawn out, and if there are doubles Ts, don't pronounce them. Say short As like short Os."
Just going to use this to show what a ridiculous question you've asked.
"British accent" you say? So that's any regional accent in any of the three countries in Britain.
Don't pronounce Rs? Scottish people and people from the West Country are rhotic.
"Say the a drawn out", presuming this is referring to the trap-bath split, only the South of England and Wales uses the long "ah" sound, so if you're going to say "bahth" that immediately places you in the South of Britain, Wales, Bristol, Devon, Cornwall, Essex (sometimes), London, RP, Estuary, Cockney etc.
"Don't pronounce double Ts", firstly, seriously? Anyone who drops Ts will drop all Ts equally, and dropping Ts, like dropping Hs is very dependent on class and education rather than accent. Even a Cockney is likely to say their Ts and Hs due to the cultural perception of anybody who doesn't.
"Say short As like short Os", again this is spectacularly unclear but I'm assuming this is referring to the American-British O sound difference. Americans say "clock" as "clahck" whereas we say it properly. This may be one of the few things that crosses most of Britain, but I'm pretty sure in the Midlands they often use American words (they say "mom" for example, rather than mam or mum like the rest of us).
The only two that you can generalise across most of Britain would be this O sound difference and U sounds, for example in the pronunciation of "tube" would just be said as "oo" in the States but be said as "yoo" in Britain. "Tchoob" compared to "toob", however this isn't across the whole of Britain, I've often heard Welsh people use the "oo" sound instead.