Dewdrop, I totally hear you on the maintenance thing - I've lost fairly substantial amounts of weight several times in the past and then gained quite a bit back.
My (private) clinic would, I think, theoretically treat me with a BMI of up to 35. I've always looked at it not as the NHS excluding people to save money, so much as private clinics maybe being a bit more prepared to take your money even when your odds aren't optimal, though. And actually, last time I visited my clinic (to plan for my upcoming IVF) they suggested I follow some specific steps in relation to diet, and try to get my BMI a bit closer to 25 (it was maybe 27ish at the time).
For my first (unsuccessful) IVF I pretty much stuffed my face with cake non-stop. This time around I've been reading a lot more about the effect of both weight and diet on success rates, and although I definitely think it's possible to get to hung up on it, I thought it would make sense to make a few changes. Partly that's been around aiming for a BMI of 25 (I'll probably be just above that when my cycle begins, but only just), and partly that's been around specific dietary changes which might potentially have a positive impact. I'm currently quite into 'It Starts With The Egg', which summarises research into the impact of various factors on fertility generally, and IVF success specifically, and has an interesting chapter on diet. I've been trying to follow the guidance there.
With that in mind, my thoughts on things like Weight Watchers or Lighter Life would be that they might not emphasise exactly the same things as what would be an optimal diet to help your body prepare for IVF (although, to be fair, there's probably a reasonable overlap), so it's maybe worth bearing that in mind? I've found My Fitness Pal helpful, because you can define your own goals, and look at graphs of your macros (really helpful for me as I'm assuming for less than 40% carbs and over 25% protein, so I can keep track of that at a glance) - also, it's free, and has a massive food database to help you track your food and exercise.