The UK may not get "really cold" (cold enough though) as such but it does get wet, wet feet are usually cold feet in the winter.
I wouldn't bother with toe covers/overshoes for offroad. I use Shimano MW02 winter boots, they are Gore Tex lined and keep my feet toasty warm - which is a miracle as my feet are always freezing.

They go down to quite small sizes as well. There are also Northwave Celcius boots which a few of my friends have and rate but I think they only go down to a UK 6/ EU 39. Both pairs of boots are widely available in the UK and both come in road specific versions as well.

Merlin have the Shimano ones for about £80:

http://www.merlincycles.co.uk/acatalog/Shoes.html

If you can its worth trying to track some down to try on as I ended up going up a size tobe able to fit in thick socks comfortably.