BritJamez:
Do you have the money?
I wouldn't touch nigeria with YOUR bargepole (which isn't saying much as you're already going there) .. but seriously - just re-list it and all that stuff.....
When you relist it remember to put "will ship to UK only" on the listing!
If you're worried about offending the Nigerian, just write them a note along the lines of:
"I'm very sorry, but when I came to post the phone I found that my postal service will no longer insure mobile phones that are sent to Nigeria. In the event of an accident I wouldn't want you to have wasted your money, and it is therefore with deep regret that I feel obliged to relist my phone with shipping to the UK only. I apologise for this mix up, and look forward to dealing with you in the future."