I wouldn't imagine any of them are preferred as they are both horrible and pointless to register as they are owned by whoever owns uk.com and gb.com (as far as I know).
Much much much better to get .co.uk or the new .uk
It depends on what You want to do. For marketing and business better use .UK.MK (UK Marketing).
F.e. Seo.uk.mk or 1.uk.mk are still available !
If its not a premium listed then ***.UK.MK is very cheap and a nice alternative for anyone who did not get his preferred .UK or .CO.UK .
Here in the UK, I occasionally see advertising with the uk.com and a great usually single keyword, but .co.uk is the norm and .uk will become more common as time goes on.