Hello man,
Best is to take your informations on specialized websites based in the UK.
http://www.gundealer.net/rules.htm
and...you will read that if you're under 18, you can't buy a shotgun.
If you're allowed to buy a gun, the second step will be the customs formalities for the return to France....
In my opinion, for an fine English shotgun it might be worth it, but other than that....buy a Verney carron in London would be very funny

...for the English!