Sorry @Ambushedbycakeinmydreams I have tried Slimming World, and found it very cliquey and almost like a cult. I hated it! I am glad you found it good and helpful though. Never tried keto.
The best way I found to lose weight - and keep it off - is intermittent fasting. I started off with the 5-2 diet, (where you eat fairly normally for 5 days of the week, and have roughly 800 calories on the other 2 days. The 800 calorie days were Monday and Thursday.)
I then changed it to 4-3 when I reached a plateau. (Lost 19 pounds and then stalled.) I found the 4-3 too much though, and felt a bit light-headed, and was more hungry. So I settled on 1 day on, (on 800 calories,) 2 days off, (eating 'normally,') 1 day on, 2 days off, 1 day on, 2 days off, and so on...........
With this eating plan, I found myself eating less on the days 'off,' where I was eating 'normally.' Eating healthily and eating enough, but not snacking on crap. And I eventually started to lose weight again (after a few more weeks.)
This way of eating is the only 'diet' I have ever been on that has actually worked. I lost almost 3 stone (40 pounds,) in 2017, and have not regained it. I still stick to the 5-2, although on the 2 days 'on' (Monday and Thursday,) I tend to have 1000-1100 calories, rather than 800.
I go swimming and walking too, and also cycling. (I go swimming twice a week, and walk 2 or 3 miles 3 times a week, and cycle 2 or 3 miles 3 times a week.) Not a massive amount, but it works for me.