It's probably to do with shipping costs. Do you have any friends or relatives in the UK that wouldn't mind you putting their address? That way you can enter and if you win anything they can hang on to it for you until you can collect it? If it's something small like a book, then they could see how much it would cost to post it to you. If it was something big like a greenhouse, I guess you'd have to fork out petrol money to fetch it.
I'm very lucky in that I'm a member of a 4x4 owners club, and we have 'Mud-Club Express', so if one of us wins something on ebay that needs collecting sharpish, we pay using paypal, a member nearby will collect it, and pass it on to someone who lives nearer, who will pass it on to someone who lives nearer still, until it's near enough to fetch. It relies a lot on trust, but I've bought several things like this, and likewise have been part of the collect/pass it on chain. It's not perfect, it can be slow, but I've never been ripped off, and always got my stuff (eventually).
I think some people are luckier than others, I'm not particularly lucky, but if you don't enter something, you have no chance at all of winning. If something's a free entry, I'll normally enter if I have the time. I ALWAYS enter the garden competitions (kitchen garden also has some great stuff they give away most months), as it's normally the only way I can get hold of stuff (unless I save amazon vouchers I get for doing surveys), or use clubcard points to exchange for stuff I need.
My favourite price is 'free'