Hey Ms BJ,
As a 2x knee surgery survivor (my years of ballet have taken a big toll) may I suggest swimming ?
I'm fighting the daily knee pain from letting my weight get above a certain point and have to lose it or risk damaging all the work the doctors have done, so I totally understand the bummer that is knee pain.
Swimming is the 'best' excercise you can do if you have any sort of joint injury. It's considered 'no impact' if I remember correctly. My physical therapy after both surgeries involved swimming and learning to walk and run again in a pool. It was great for weight loss and I was able to do it without any further strain on my knees. Since gyms can be expensive and not all english houses are roomy enough to invest in a stationary bike or weight machine, I'd suggest finding a pool at a local recreation center.
I know public pools were in abundance where we used to live (Nottingham) and the memberships are fairly cheap. Even if you just start out walking or jogging in the pool you'll notice an improvement pretty quickly.
Now unlike Shel I have to fully wrap my knee before a workout, but that's because my knees don't lock, instead the knee cap will slip out of place. So depending on what your knees do, you can also invest in brace or a bandage....Boots carries them. Heat treatments after your excercising will always help with fluid build up or pain.
Good luck to you !
Red