The above video shows quite clearly that shot placement is everything.
We all know that the firearms rules across the UK are totally irrational and inconsistent from one county to the next. I certainly wouldn't ever advocate shooting a boar with such a small calibre as a .17HMR, but as others on here have said in other countries smaller calibres are legal and very effective if the bullet is placed on the right spot. Countries like USA, Canada and Africa do a lot more big game hunting than we in the UK will ever do. I think we need to listen to what they have to say.
IMHO we need to spend less time worrying about calibre and more time worrying about hitting the right spot.
Good point about shot placement, but the shooter is complete nitwit to have tried it!