Hey Jack,
There's a few options..
Fit some UK spec shocks and hope for the best -
Clicky - I did this a few years back with no hassle. Paired them to a set of lowering springs to make sure ride height wasn't affected.
Fit upgraded shocks, for example these from Demon Tweeks -
Clicky - Might be worth asking them if those are just inserts (bit of messing required) or complete shocks first though.
Find and fit a random used shock -
Clicky - Bit of a gamble but might work better for you as they look to be plug and play.
Worth noting that good engineering practice is to replace shock absorbers in pairs in case you unbalance the vehicle's dampening. I've had a few guys recently buy one shock and then come back after to find it's mate from the same car.
Also check the above part numbers with your local motor factors. They should be able to bring them in slightly cheaper.
Hope that helps,
Jay