If we can invade Afghanistan and Iraq, why can't we invade the Channel Islands? The Germans did so, and it was a cakewalk.
There is a very good casus belli: the Islands leech billions of pounds of revenue away from Her Majesty's Revenue and Customs. It's really all they do - growing potatoes and issuing postage stamps of the Queen Mother is just PR window dressing.
There is one small problem. The Channel Islands (and the Isle of Man) are Crown Dependencies. We gave them their Devo Max status and we gave it to them precisely in order that they could provide a haven to those for whom paying taxes has always been a voluntary matter. Take a look at Nicholas Shaxson's book, Treasure Islands. Parliament wouldn't be Parliament if it didn't provide the wealthy with legitimate opportunities for tax avoidance.