Yes, I have worn cartilage, although no bone on bone yet as far as I know. What you need is a tailored exercise programme from a physio to make sure all your muscles are working properly, especially when cycling. In principle cycling is great as its low impact, but make sure your bike fit is optimal, especially saddle height and position. Otherwise you could do more harm than good. There are plenty of exercises, but you need a physio to show you which ones are right for you and how to do them. It'll be worth spending a few hundred quid on some sessions with a sports physio, preferably one with interest in cycling. Also, get used to stretching after cycling if you can. Having the correct gearing and spinning not grinding, making sure your cleats are in the right place etc are also key
Swimming wise, frog kick in breaststroke is not usually advised especially op if you have worn kneecaps but see what feels most comfortable.
Where in the knee is the arthritis, how severe is it? What has the specialist said in terms of treatment? I find hyaluronic acid injections in my left knee help. I do them privately, but you may be able to wangle on the NHS
The other really key thing is keeping your weight down (not that I am much good at this).
Hopefully you've caught it fairly early, however a knee replacement isn't the end of course, lots of people get huge benefits from them (how widespread the damage is and your age will likely determine whether one is done)
