For two people and a pet this is very high. In short, medical costs aside I'd suggest you are still living as though you are still in a highly paid job full time.
£50 a month pocket money and another £50 is not extortionate for a teenage boy. Is it worth having a conversation about cutting the cleaners hours and sharing some of the savings if he/she is difficult to motivate?
If you are seeing a therapist - cut back by 50%. It doesn't have to be forever, just until the house in France is rented.
If the internet is to be believed, everyone in the USA wants to emigrate right now and they can visit for up to 90 days on a holiday visa. Rent it out and be careful to set the rent appropriately if they will expect a local mgmt company to sort any maintenance issues. I generalise but they don't seem terribly low maintenance in their expectations and you don't want to make it more stressful. What is interesting is that if American's leave the Schengen area [which UK is not part of] for 90 days, then can go back for another 90 before having to leave fully. So if you found the right candidates, you could rent France out until school hols and spend the summer in France and swap over. If you haven't heard the term digital nomad, look it up.
£500 on food is high for two people especially if eating out too. Is that a lot of Deliveroo? If it is, then I'd do some calculations and if you are spending £200 on pizzas every month for a teenage boy and visiting friends then I'd load the freezer up with nice ones instead. You'll still save a fortune and they'll only have to wait 20 mins. Bonus, you teach a teenager to be a bit more useful.
Look at how much food is being wasted if a lot of that is fresh food. Plan your meals or shop on the day only. A bag of salad here and there in the bin doesn't feel like much but clean out your fridge today and work out how much you'd bin by the end of the week if you don't eat it all by supermarket day.
Is your freezer loaded? Start an economy drive and work through it. Mission full defrost commence :) You want nothing but ice and vodka in there by July [except pizzas obviously] so you can swan off to France for a nice holiday and continued recovery.