I see it as a green/eco campaign site - what green initiatives are being followed round the GB & N.Ireland.
I like the G and GB alliteration so it has some brandability.
You could put stuff on there about solar panels, etc, and other green consumer items to get some ad revenue.
Have you regged the .org too? Might be worthwhile. However, the .com does a good job as GB figures in the name itself. I wouldn't use the .co.uk - just point it at the .com.
Value? Dunno - but build a site there and I think it would do well.