Depends on the market IMO...
For purely UK :
1. .co.uk
2. .com
3. .org.uk
For Global :
1. .com
2. .net
3. .org
(But to be honest - I probably wouldn't ever buy a .org unless a really REALLY good generic and I wouldn't jump at buying the .net unless there was no way I could get the .com.)
For branding as a UK company
1. .co.uk
2. .com
For
SEO
1. .co.uk
1. .com
1. .org.uk
(Really makes very little difference in my experience what you use if you have the right keyword domain, content, IBL's and online strategy. But as traffic kicks in I would prefer .co.uk and .com over .org.uk purely due to recognition by dimmer members of the public at large)