You are definitely going to find London beer prices a bit of a shock, so bring plenty of money.
The amount you usually get as a "taster" can sometimes be too small to make a correct choice, especially as you are pressurized to make a quick decision while standing at the bar.
My local in Birmingham, The Old Joint Stock, always has two separate barrels of our favorite beer on the go, so SWMBO and I get a half pint from each pump, then go sit down and decide which barrel to help to empty!!
Old Joint Stock pub with great food, good beer in an amazing character building with its own pub theatre.