There is no reason to try and obtain rubles before coming to Russia. As another person said, there are ATMs everywhere. Just use your UK bank card and take out the desired quantity of rubles. They'll charge you a small fee (1% or less).
If you have cash, it's best to bring dollars or Euros rather than sterling. There are little exchange places all over Moscow. Their rates are generally very good--certainly much better than the rip-off one experiences in London. Still, my advice would be to just use your ATM card--why risk travelling with a lot of cash when it's not necessary?