It seems that for a new bike, 500 pounds would afford a *very* nice single-speed and a not-as-nice geared bike, of course buying a used machine would change that significantly. The OP did not specify his location. How hilly are the rides around town, and how young are the knees?

OP mentioned Canyon rigs - nice, and seemingly pricier than the 500 pound budget.

I have eyed the VanMoof cycles, but their only dealer in the Eastern US is in New York, I see they have several dealers in the UK. The "Standard" bike with chain guard, fenders, and integrated head and tail marker lights retails for 485 pounds, which is pricey for a single speed, but appear to be a cool low-maintenance machine with useful accessories. A 3 speed Standard is a bit more spendy,