8500 .... is a great deal for a .co.uk. Go for the cash...try to get 10K outta them they are obviously interested.
The lease is a hassle and they could end up breaking the contract later down the line.
Do the lease if at the end YOU want to use the domain to develop a site...
this way they will have spent the time and money building traffic for ya : )
IF you do the lease make sure YOU have a lawyer draw up the papers with your terms and your language.
You don't want to find out after you've signed a contract they supplied that if you wear plaid on a sunday in may, the ownership reverts to them...( people put strange stuff in contracts )
IMHO