Look in the Yellow Pages and the Thompson directory for rescue kennels and animal charities. Phone around and you will make so many contacts with homeless kittens you will be amazed. Most UK shelters won't admit to, or release animals until a few days into the New Year to avoid the problem of pets being given as presents so don't take offence and try again in a week or so.
Again, at this time of year it is rare but try the local newsagents, supermarkets, pet shops and vets for notices of litters.
Good luck and let us know where and how and what sort of cat you end up with.
(It is really worth waiting a couple of weeks to phone the shelters!)