Modifying in the UK has an insurance impact, which is not the case in continental europe where you only insure the value of the car, no matter what you do to it as long as it is legal of course.
This obviously has an impact on the resale value. The good thing about the Alpina is that they are different, and rare. That will increase the value a little compared to the standard models. If you compare that one at 20k, a standard 3.0 would be 12K for the same age, so the original price difference between the two models still exist, so as an investment it is not a bad deal to have bought one of those.