I would steer clear of uk.com, because it's too easy for people to confuse it with .com or .co.uk and end up at your competitor's site – so on that basis, .net is a better choice. Likewise .co is in danger of being misread or seen as a typo, so wise to avoid it. The other danger of TLDs like .co is that one day Colombia might decide it doesn't want the entire world to be using its domains, and start charging lots of money for you to renew – we've recently heard of one country that similarly gave out its domains freely and has now been charging thousands of pounds for people outside the country to renew. That's a scam you don't want to get caught out by.
In terms of SEO, Google couldn't care less. As long as it can identify what country your site belongs to (which it can do even if you don't choose the right country-specific TLD, if you set a location in GWT and everything else sings the same tune), the TLD is irrelevant.
Note that above I said that ".net is a better choice", but I didn't say it was a good choice. Unless the other sites that have registered the .com and .co.uk domains of the same name are in a completely unrelated sector, I would always try to avoid going head-to-head with companies that have an existing online presence at a more memorable/recognisable form of the URL. There's too much of a risk that people will type the wrong one in. Far better to change the domain name slightly so that what you have is unique, and that way you're less likely to lose customers to your competitors.