.co.uk is a UK domain name, whereas .com is an International domain name.

Both are accessible from anywhere in the world and there is no performance difference between the two. The main difference is in that .com's are perceived as being more international and .co.uk's are seen as being UK based.

So if your company has an international market you may prefer to go for a .com, but if your market is mainly UK based or you operate mainly in a local area you may prefer a .co.uk.