Pooch. Until measured, you don't know if any harm has really been done. It is completely acceptable to lengthen the chambers in the British system, is is unacceptable to have the gun returned without being subjected to reproofing at London or Birmingham.

There is a Brit (smallbore) named Diggory Hadoke that is in Texas sometimes, he could facilitate this gun being sent back to be proven in the UK and he'd tell you quickly if irrevocable damage has been done to the chambers.

There are many very valuable vintage guns out there that are proven for 3" modern magnum shells. So long as they are reproofed, it is a feature, not a detriment.