Hi OP,
I understand where you're coming from. In my 20s and early 30s all was fine and dandy. By the time I got into my early 40s, I'd gone from around 10 stones to 15 stones 
I felt awful and, to be honest, was quite miserable.
I joined a weight management club as I felt that I needed the group support. Purely based on location, I signed up to Weight Watchers and .... I love it. I've lost six stones and now weigh in at just under nine stones. I did it in phases: so I'd lose a stone then maintain that for a while; rinse and repeat so to speak. It took me a whole seven years to finally hit my target but don't worry: that's not typical and most people would do it a lot quicker.
So Weight Watchers ..... It basically works on a points basis. Each food has a points value (we call them SmartPoints); you get an allowance each day, plus some extras each week for your weekend wine or whatever your choice of treat is. Fruit, vegetables and lean protein (chicken, fish, eggs and so on) are free; however that means that a sensible portion of those things at meal times counts as zero points: you can't eat unlimited quantities (everything counts eventually). Your Coach would explain all that to you at your meeting.
I still weigh in each week. When push comes to shove, I'm and over eater who's recovering: I'm not cured 
I love Weight Watchers; it worked for me (I've posted my before and after pics just in case that helps).
I don't know anything about Slimming World as I've never been but I know that it also works for a lot of people. I'm sure someone on here would be able to give you the lowdown.
Lots of luck and please update and let us know how you're going on 