I've had that question numerous times, but vast majority of my shipments weigh 3oz or less thus no form.
ebay has their rules and buyers have to abide, but in the end it's up to you.
Still no reason to block UK as you'll still be cutting off the nose to spite the face and even your domestic sales will drop in value as underbidders can also be international bidders. Your bottom line will take a beating far worse than the occasional mishap.