My understanding is thus (but may be wrong of course!)The Uk ex-mil Gazelles were modified to such an extent from the original machine that they were no longer able to be classified as SA341's, hence not able to be put on the UK register (Only SA341 is CAA approved). There are a few French registered SA342's operating in the UK privately, which may have a similar spec. As for operating UK P.T. Category - don't hold your breath. Permit to fly may be the limit for these a/c.
As for buying / running one, rumours abound concerning spares from Eastern Europe, so take care. I have dealt with our Eastern European cousins, and they don't all share the Uk view on flight safety!!