Viper eludes to the answer I think - it's a combination of its physical location, and the air masses it's exposed to. It's land locked - the closest large areas of water are the Black Sea and the Caspian Sea, but neither are ocean sized, so the warming effect an ocean would provide in the winter months is not prevalent. The prevailing air masses coming down from the arctic or across from Siberia will start cold and not be heated by the land either.
In short, pretty cold. Not horrendously (on average), but pretty cold.
PS. London isn't terribly wet, but the UK has a fair bit of rainfall as our prevailing air mass comes from the atlantic; if we're lucky up from the Azores, if we're unlucky from the arctic or Newfoundland!
