Others are better qualified than me on this, but the CAA restrictions include max 4POB for starters (and if memory serves, some odd clause about no kids under 16??). Hence the need to look elsewhere to get 5POB in a machine made for 5POB....
The main difference is that the ending is a different engine - the ex-mil ones having an Astazou III while the SA341G civil model has the Astazou XIV. Even odder is that the CAA do not allow the SA342J/L models which have a marginally longer cabin and the same XIV engine that they are happy with in the 341G model.
With MW involved in five French registered Gazelles in the UK (and that interest dating back to May 98), I am also wondering why the option of putting these ex-mil helicopters on the F register has seemingly been rejected.
Even more interesting, the first ex-UK-mil Gazelle has now appeared on the FAA register.... and the FAA regaards the type of helicopter as having 5 seats (check landings.com for reg N911XW if you like..)