I'm not familiar with Spire Southampton, but I would check it out, read reviews etc. If you can get a quality package for just under 500 which includes a dietitian and food related counselling for 12 weeks this may be a good deal.
I agree with @Peridot1 , especially with regard to Concentrating on whole foods, lots of fruit and veg and whole grains and protein. Cut way back on any processed foods. Ignore the Muller Light and other low cal processed rubbish.
When I was on a diet, I aimed to eat 1300-1400 calories between 10 am and 6 pm (intermittent fast) and I cut out all refined carbs, sugar, alcohol and UPF. Eating mostly the following I was never hungry:
Brunch: 2 egg omelette with vegetables or mixed salad with 2 boiled eggs or a tin of tuna
Snack: 2 tablespoons of yoghurt or cottage cheese plus a cup of berries or a handful of nuts
Dinner: LOTS of vegetables plus c. 100-120g of lean protein (chicken or salmon mostly) and half a cup of lentils or quinoa.
I would make up a batch of salad dressing consisting of lemon juice, a bit of a balsamic, EVOO, mustard, salt pepper, so I could be sure to add only a teaspoon or so to my salads. I'd also make various marinades to flavour the chicken.
There's an excellent book by Dr Mark Hyman called The Sugar Solution which was a total eye opener to me. I also find Dr Becky Gillaspy on YouTube very good at explaining healthy dietary choices.
Can you also consider exercise? Walking is always good, but Lucy Wyndham on YouTube has lots of short and not very challenging workouts that are perfect for beginners. She is very calm and gentle - not at all shouty like some online trainers.