Ok, I ground my way from 25NL and 50NL to 100NL, where I'm still grinding!

You haven't given any indication of the number of hands/week you'll be playing, but I go along with the standard advice which is not to move up until a) you've beaten the current game for at least 4PTBB/100 over 20K hands, b) have at least 25 buy ins (maybe more) for the new level, and c) feel you're significantly better than most of your opponents. Your idea of moving up based on BR only is far too risky.

If you withdraw from your BR, it's obviously going to take longer to build the 25 buy ins necessary to move up.

Bonus whoring is the way to go. Check out (if youhaven't already) what's available at bonuswhores.com, and as a Brit make sure that whichever site you play at has sufficient games at the right times for you.