Sergeant Pluck's comments mirror my experience and observations.
Carrying too much weight around is going to put extra stress on all your joints, regardless of what anyone says about "fit and fat". I've seen too many overweight/obese coworkers, family members and other acquaintances needing joint replacement surgery to believe otherwise. Your weight issues are not as severe as the people I'm thinking of, but I still see a correlation between being overweight and experiencing mobility problems later in life.
Food: again, I agree with Sergeant Pluck. Calorie counting might bring slow to almost nonexistent progress, but it will be progress, and at least you won't be likely to gain more weight. If nothing else, you'll save a little money on food! Part of my small progress in the last year or so has come from my modest efforts at portion control and avoiding too many processed foods.
You mentioned walking 4-5 miles a day at work, and being in pain sometimes at the end of a work shift. Are you familiar with Superfeet insoles (
https://www.superfeet.com/en-us/why-superfeet)? A number of people on this forum use them, so I assume that they're available in the UK. I've been using them for most of my adult life, and I can feel the difference when I leave them out of my shoes for a day.
If you're going to run, please do it on unpaved surfaces, not concrete or asphalt. Paved surfaces are hard on the whole body, especially the knees.
Spinning classes with a good instructor can be a lot of fun, but they can be brutally hard. Find out beforehand how demanding the class is meant to be, and don't be afraid to go at your own pace.