From the lateral threads and posts answering your original question, there are guys who have hunted boars and lots who haven't. I'm in the latter group, so my answer is only a suggestion.
A 12ga shotgun with slugs shouldn't be a problem in UK. Nor should a full-bore hunting rifle like a stalker would use on Red Deer. A military based cartridge, like the .303, 7.62, 30-06 or 8mm Mauser should be easy to find and feed. After that the exotic European guns seem like a good idea, followed distantly by the North American cartridges.
I think solid round nose bullets will give you penetration and energy dissipation to knock down the beastie.
From you Flash Earth map (great site, thanks for the link), you have multiple tiny woods and fields, with a scattering of isolated farms and hamlets. It would be very uncivilized to let a wounded animal escape to terrorize the citizenry. Your feet are probably nailed to the particular plots where you have permission, and won't be allowed to pursue and dispatch wounded animal. For that reason, I think your justification should be 'bring enough gun'. If the local cops limit your muzzle energy and terminal velocity, it might be worthwhile to point out that they will have to respond with a state of emergency and mobilize their armed squads (with peashooter guns too). (Refer back to your comment about arguing with know-it-all policemen.) Doing it right the first time makes more sense.