If your Sim Assessment is on the -800, why not get a few hours on an -800 ?
Doing a few hours on a -200 might not be the best way to spend your money. I presume you're not looking to practice flying a SID, you can do that in an FNPT or on your iPhone for that matter, what you're looking for is a feel for the sim you'll be checked on? Not sit in the sim for the first time on test day saying to yourself "the -200 didnt have those buttons' or "the -200 wasn't this fast".
It might cost a few more €, but its probably money well spent if you can find somewhere to do it.
However, the one piece of good advice I can give you is dont discount any advice (including post #2). What are they looking for in a candidate? Knowledge of the -800? Or perhaps how quickly a candidate can pick it up, retain the information they're given, and adapt to a new aircraft?
Good luck anyway.
(And I too would be interested in any feedback from those with experience of VirtualFlight in Gatwick).