Oldest British Boxing champion for British belt

Yes in modern times. Jem Mace was the oldest ever, or at least thus far.

 

But didn’t win a british title technically as it didn’t exist then.

 

It depends what you call a British title. Different eras have different standards. I think most fans of his era would have called him a British champ, but you are right for this era, that does compare to Andries claim.

 

Also, Jem was pre Queensbury Rules although, according to some, they were stolen from him.

 

Dennis was a really awkward customer. Strong and as brave as they come but his fights were not, for the most part, things of beauty. He mauled and clinched and wore guys down so although he is hugely respected by people in or close to the game he really didn't have cross over appeal
I would though recommend you watch his fights with the Australian Jeff Harding. Utter classics.