It is illegal to sell an out of proof gun in the UK, but you can give them away, consequently an out of proof gun is avoided, unless you really know what you are doing, buying (begging?) for spares, or you are going to re-manufacture, a well done sleeved barrel set is perfectly usable, after all it has to be re-proofed, but it does devalue the gun in the region of 30%-50%, so ask yourself do I want to invest money in this project.