There are two main options for exchanging cash currency, and they are using banks or using money changers.  Banks will certainly undertake the service for you, but they will generally charge you more and it is highly likely that you could be up for a long wait to get to the bank cashier - don't anticipate anything less than 30 minutes, unless you find a quiet bank branch.

Money Changers on the other hand are all over Moscow and they will generally offer better rates than the banks and there will not be long lines to use one (and if there is, you can walk on to the next one).  They all have signs showing where they are and their signs will also show the rates that they are offering - both buy and sell rates and the currencies that they deal in.  Every Money Changer will deal in USD and Euro to Ruble (but they will not always have enough Euro to give you back Euro for Rubles) and a significant number will also deal in GBP.  If they deal in GBP they will usually have that on their sign with their rates, or if not, you can check inside their booth where they will also display the rates and currencies that they deal with.