is a co.uk a disadvantage outside the uk.
generally, anyone doing a Global search and getting all results will find the .co.uks appearing beneath a .com in the SERP, so, the answer is yes.
I would suggest that it would also depend upon other factors:
1. the subject matter
2. competitivness of search terms
3. whether the surfer really needs a supplier for webdesign in the USA when they might me looking for webdesign in Clacton-on-sea.
Depending on which SE's generate your "money" traffic I don't believe that having a co.uk is necessarily a disadvantage (some of our co.uk results beat our similar com pages on US databases but there are other factors at play).
Back to oLeon's question, if you're after UK traffic then you must have the co.uk domain.