Educate me please.

I'm on a waiting list for a Weihrauch HW 100 .22 and was informed by my dealer that this gun will likely be de-tuned due to restrictions in the UK.

He suggested that I turn up the regulator right out of the box then chronograph it and adjust from there. 

Shot count may suffer but performance gained is the claim. 

I was unaware of restrictions across the pond. Can someone give some insight on this subject?
 
I believe the opposite is the case. You would have a hard time finding a 12 fpe rifle in the US. Not to be cynical, but it's possible your dealer has a 12 fpe version, needs to move it, and is trying to make it sound like the standard. I had a carbine version HW100 in .22, and I believe it was shooting about 27 fpe as delivered. I expect the rifle was designed on the European 12 fpe platform, so shot count is not terrific, but it's a great shooting rifle.