There is an area of the auction where you can specify what regions of the world you will ship the item to - select "United Kingdom" only.
As a double security measure, also make it extremely clear that you will not send the item outside of the United Kingdom within your auction details - something along the lines of "Will only ship to the UK. Will not ship anywhere else - especially Nigeria!".
If you do this, and some numpty still tries to scam you, simply report them as a non-paying bidder, I forget the exact reason but there is one specific to "buyer wants item shipped to a region not offered by seller" or similar - choose this. eBay will look into it and suspend/delete the offending account and refund any fee's your out by.
I've had this happen on two of my listings, and in both cases have had no problems whatsoever with how eBay have dealt with it. Usually only happens on high profile/high value goods - things like phones, computers, TVs etc. Although increasingly, any item is being targetted more recently. For the scammers, it's a case of "lets bid on 100 things, one idiot out of that lot is bound to fall for it".
I work in eCommerce as an industry, and the amount of fraud through all facets is incredible, and currently there is virtually no defense for the merchant/seller, it all favours the buying "victim".