Ello welcome back into the fold! I'll take your questions:
I need to devise a basic menu which I can work with the family.
Do you / your family mind things being a bit repetitive? We eat the same things over and over in this house (during the week especially - I get more creative at weekends or when we have company, but not much more) purely because I like to keep things simple. I keep a spreadsheet of all of our meals so I can just flick my eye over it if I'm feeling uninspired, but also so I can pre-enter our dinners, which I do as I'm doing the weekly shop online, to make sure we have everything. Takes the thought out of it and, more importantly, the spontaneity. Husband has the same breakfast every day and packed lunches which are essentially the same 3/4 things on repeat. I'm very happy to help you come up with a meal plan for a few weeks whilst you get into the swing of it (and I'm sure the more creative others will too!) then it's just a question of keeping it ticking over.
Is 800 too low? 1600 to high?
Not necessarily and not necessarily #Helpful I would say that 1600 is almost certainly too high for you. It's unlikely to leave you in a calorie deficit unless you are in an exercise regime. There is no "too low" on any given day; plenty of people have 500 cal days, and I do a 48hr-ish liquid fast each week. But the right calorie amount is what you can stick to the majority of the time without being hungry, grumpy, miserable, etc, and 800 is tough psychologically (even if it's adequate physically). Most people find 1200 to be a happy medium: doesn't allow for many indulgences but doesn't leave you hungry or deprived. There's a great reddit feed called something like "1200 is plenty" where people post their 1200-cal daily meal plans, great for inspiration!
Do I have to do the gym or can I just do something I like?
Of course you can do what you like, you great noodle I haven't been to the gym in donkey's years. I like plugging into a podcast / audiobook and going for a walk, so I do that and nothing more. It keeps the weight off, and I'm not toned, but I don't seem to care enough about that to do anything about it...
Can any cereal actually be healthy? I'm yearning for fruit and fibre in the morning at the mo, eggs don't always appeal.
Boxed cereal can't really ever be healthy. The problem is that they're stuffed with sugar; and also that the calorie amount cited on the box is usually for 30g, and I think most people naturally have about triple that! 30g is absolutely measley. Do you like porridge? 30g of sugary cereal simply doesn't leave most people satisfied all morning, so you get a lot more bang for your buck with porridge or protein. That said, it won't kill you to just have it when you don't fancy eggs a couple of times a week.
Regarding the exercise thing: is your rationale that if you've paid for it you'll be motivated to do it? I know that works for some people, but I wouldn't trust myself not to just waste money and then hate myself. I don't know anything about that programme, but my suggestion would be to committing to doing a free youtube tutorial x times a week for a fortnight first, see if you can do that, and then reassess. Even if it's not your ideal workout programme, find one or several you don't mind too much and do it just for the habit-forming of it.
Blahblah 3kg in 1 month is a tall order, we'll handhold you through it! What an amazing achievement so far, well done. What's your strategy been?
Kick it's the weather, definitely! As Ello says everyone wants to hibernate and gain a winter coat It would also make me disinclined to go for a run, but then, I am always disinclined to do that, you might feel different.
Here's my shout: if you have an absolutely superbly golden day today and tomorrow you'll feel AMAZING, in control and determined, and it'll carry you through the indulgent bit and you'll come out the other side clear-headed and ready to tackle any gains. If you admit defeat now and just eat your way through it, you'll feel sluggish and grumpy and will struggle even more to find the wagon.
So start now, and then enjoy your plans, and then get back on it. You know it makes sense!