A gun can be "out of proof" for reasons other than bore size or chamber length.A gun with a split in the barrel or a cracked action is deemed out of proof . It is Illegal to export a gun form the UK that is out of proof no matter how many parts you send it in. It is illegal to export a gun with out export papers.To sell it as a stock, action and forend is Ok if you want to include the barrels in that, then they must have a section cut out of them at 9 inches.
That's it. No way round it and now as many people know of the problem with the gun it would be most unwise to try.
Your choices are to either deactivate it , scrap it/put it in auction as stock action and forend ,or sleeve it .