If UK companies are working locally or nationally they tend to prefer the .co.uk but if they are working internationally the .com is probably better.
Be aware that it is much easier to transfer a dot com name than a .co.uk (for which all sorts of paperwork and a fee was involved) so if you are new to domaining I would stick with the dot com names for the time being.
I think Nominet brought in a paperless online transfer system last year to replace the old, prohibitive paper system. Not 100% on that mind you.


