Can you post pictures of it? The only slight problem is that it is in theory so impossible as an error, that any potential buyers may: 1. doubt its authenticity or, 2. consider it as a coin a mint employee made for a laugh! No. 2 is possible of course. It is unusual and possibly unique, but don't be too greedy as UK errors are historically pretty cheap. You may find someone for a couple of hundred quid, but I don't think we're talking thousands.