Haven't used the lens, so can't comment. However, the price seems awfully high. B&H in New York has it for ~$5300 USD, which should work out to ~2650 pounds. Is there a reason people from UK don't buy from the U.S., like prohibitive shipping or tariffs?
How the costings go (using vague guesses for rates/amounts):
USD 5300 -> GBP 2650 at central bank rate
+ 100 (?) shipping + insurance -> 2750
+ 1% (?) currency conversion = 27.5 -> 2777.5
+ 10% (?) import duty = 277.75 -> 3055.25
+ 17.5% VAT (sales tax) = 534.67 -> 3589.92
+ 20 (?) carrier's handling charge for collecting duty/tax -> 3609.92
So you could save just shy of 10%.
On the other hand, you don't get to handle the lens beforehand, there can be warranty issues, legal issues in the case of a dispute, do you have to return the lens to the US for service/fault repair? etc etc.
There are assorted tricks to try and avoid some of the costs, but a) HM Customs & Excise have seen them all before, and b) if you get caught, it will end up costing you considerably more than the UK price, and possibly a criminal record too.