I assume they still want to use the house? Hence why they are planning on paying you rent. They could just gift it to you outright, but then they wouldn’t be able to use it themselves legally (or at least in terms of tax planning)
It is a definite viable inheritance tax planning strategy. They haven’t just made it up from nowhere! However they, and you, should take proper professional advice. Especially considering the numbers involved.
As far as you’re concerned, you’ll gain an 800k property and 2k per month income. The income will be subject to tax. You will also have to register as a landlord etc but that’s not particularly onerous, and it’s not like you’re going to have nightmare tenants! Presumably they will cover things such as stamp duty on the sale, but those costs need to be considered.
They will have to pay capital gains tax on the transfer. That is (currently!) around 20% tax on the gain on the property. If they live 7 years there is no inheritance tax.
If they don’t live 7 years (or don’t do the gift) then when they die the property will be subject to inheritance tax presumably on its full value at (currently) 40%. So yes a big difference potentially!
This is legal. Whether it is moral is another discussion, but it’s well within sensible tax planning rules and isn’t remotely dodgy. Of course the new budget could change it all, but noone knows what’s going to happen there.
Deprivation of assets for care home fees etc is completely separate matter and has different rules. Although it does sound like that are wealthy enough for this not to be a concern anyway.