If I want to put a colon after U.K.

Is it

U.K.:

or

U.K:

??

Or something different?

I would go with U.K.:

or try to omit need for the colon - "...lots of dogs in the U.K. Breeds include:"

I would go with U.K.: seeing as both U. and K. are abbreviations. So it wouldn't make sense to signify this with a dot for one but not the other.

How about UK:

Checked my Oxford A-Z of punctuation and it says if you are using the letters to represent words there is no need to use full stops

its in a title, so I have to have the : straight after the U.K.

I prefer UK, but sup has gone through entire thesis demanding U.K. instead :-s