Dot coms are a lot harder to find so it might be easier to find a co.uk and also it the site is mainly for the uk it would be easier for people to remember the co.uk domain since that is what they are used to.
I normally use godaddy since their interface and tools are so easy to use to manage the domains with
You can buy anything now, .com .photos .info you can go on and on, a lot of people buy the .com now because that is kind of the standard. Although if you are from the UK it makes sense to use .co.uk, I am looking to be moving to Australia next year so i purposely went for .com as it doesnt tie me to the UK when I move, .com is more global