.co.uk is dominant in the UK.
.org.uk, ltd.uk, plc.uk, .me.uk etc are lagging far behind.
As for .org.uk it usually means it is actually used for a non-profit organization of some sort. It's less 'diluted' than .org but not very commercial in nature. If you won't use it, stay away IMHO.
Only the creator could tell you about the inner workings of Estibot, personally I would not decide my purchases based on it.