Apparently people didn't need to do anything official - just sort of let it be known, or simply start using the new name. Some I was researching did that, and a right tangle it led to! There is the instrument known as DEED POLL, and I seem to recall that the London Gazette published those,so it may be worth searching there. I spent ages searching for marriages for a female, only to realise she simple slid into "Mrs....." whoever she was with at the time. Slow diligent elimination was the only way to narrow down what had happened. Good luck.