To date, the CAA has been prepared to issue ex-mil Gazelles with only a restricted Permit to Fly. It is very unlikely that the CAA's stance will change.
Although it is theoretically possible to do the mods necessary to comply with the requirements of a C of A, the total cost would exceed the cost of simply buying a civvy model.
Provided the restrictions of the Permit won't be a problem, buying an ex-mil version gives you a superb helicopter at much less than the cost of a civvy model - but check the restrictions before you part with any money.
RAF and RN Gazelles have a stabilisation system (SAS) which makes the helicopter easier to fly - when you get used to it. Few Army Gazelles have SAS.
The Gazelle specialist North East of London mentioned by Fat Freddy's Cat is MW Helicopters at Stapleford. If you're seriously interested in buying a Gazelle (either civvy or ex-mil) Martin Woods is the man to whom you should speak.