Hello Yabba. You've already gotten lots of good replies here, and I want to offer encouragement in a bit of a different direction.
I think we're about the same age. I'm also peri-menopausal, and I likewise seem unable to just do my same-old LC thing and magically be 130 lbs. It's sucks and sometimes feels unfair.
I think you have two big obstacles here: the first is not accepting that our bodies change, period, especially with age. We ladies are supposed to be curvy, no matter what social pressures the media puts on us. You're tall, only a tad overweight, and I'll assume otherwise healthy. This should not have the capacity to be soul-destroying.
Look at those hot ladies in the plus sized swim suits on that website. They're gorgeous! So are we. They're also younger. They'll get their turn with the cellulite and sagging.
I think your second big obstacle is feeling like this is such a sacrifice, such an effort. Can we get you happier with your food and lifestyle? Another member (can't remember who, sorry) recently said, "I eat like a queen." I could not possibly agree more. I've been doing this for 18 years and I still feel excited about my food. Today, I'll be eating shawarma chicken over a Greek salad with homemade feta dressing, and I'll be picking my first zucchini of the year from my garden to turn into "zoodles" that I'll have tomorrow with a meat sauce. I actually get a bit sad when I'm not hungry yet for my awesome meals.
I don't know how it is in the UK, but here in Canada, I can even go to just about any pub or restaurant and find something suitable; and not feel left out or sorry for myself.
Remember: we're the ones eating the normal, healthy human diet.
There is zero reason to be jealous of people eating "whatever they want", even if they aren't having obvious medical consequences. You never know what they're dealing with privately. Thin does not necessarily mean healthy.
You don't "need to" lose a stone. It might be nice to lose a stone. You "need to" feed yourself good food, keep getting some exercise that you enjoy, try to get sufficient sleep, have good relationships and keep stress as low as you can. The results thereof are largely out of our control. Be proud of yourself. You are a success because you're doing it.
Since you asked specifically about stall-breaking, I'll just add (to the above) going completely grain-free if you haven't already. It's tough because some foods are more or less inflammatory/insulinogenic in different people, and you still have to find a lifestyle that's livable. Perhaps you'll consider keeping a food log in your journal. You don't necessarily have to weigh and measure everything, just jot it down.
I have varicose veins, cellulite, a beer gut, bat wings and thinning hair. I rock long skirts and cute tops that show off my slim arms and sculpted shoulders. I have burn scars on my arms from work, but I don't care; I earned them. I don't like wearing a bathing suit, but when I do, it's a cute one sort of like the retro ones above. It has a little ruffled skirt and the top shows off my, ahem, assets.
No one's going to stare at your legs. YES, I'm self-conscious about imperfections and wonder what people think, but... guess what:
No one cares.
Now you go on that vacation, laugh, eat, dance and live your best life.