I have some, limited experience with .net and it is proving hard to rank. I know .co.uk ranks well. Which is best?
Both are Good domain name extension. if your business or website located in Uk than .co.uk extension will be helpful regarding your website local SEO.
It really shouldn't make a difference as far as ranking goes. The only time is if you perhaps had a UK centric site, didn't set UK as target country in Google Search Console and were hosting in the USA (or rather not in the UK). Google would then make best guess and choose the host country. If you have a .co.uk then you are more likely to show in a google.co.uk search (if the above scenario is true)