There is no requirement for minimum wall thicknes in the English rules of proof.Provided the barrel bores or chambers on your gun have not been enlarged and/or the exterior of the barrels have not been refinished[ie;metal removed to reduce wall thickness] your gun is still in proof and legal for sale in the U.K.
The major risk with thin barrels is that they ar easily damaged[ dented] if this occurs there is little chsnce that they can be repaired,leaving sleeving as the only realistic repair option.
I would also recommend reinspection of the barrel wall thickness after first making sure that the dial indicator used to check size is verified for accuracy by checking with with a master of known size.
