Anything that's not nailed down...
This, i wouldn’t leave something like a nest (assuming I had the original bits, you would need to leave the heating in working order) or CCTV system or bathroom mirrors if they could be reasonably removed. That sort of stuff could cost £1k+ to replace.
Basically you can take anything that isn’t on the inventory. In fact you should really take everything that isn’t. Most people take a view to thing like curtains and loo roll holders when they fill out the forms. If they don’t want it or can’t be bothered to take it down it gets put on the form. Otherwise it’s taken. It’s up to you to accept the inventory as a part of the contract. You are not obligated to accept people leaving certain things behind.
You should be guided by what’s on the form, you aren’t obligated to leave anything (including the insulation!) but you should expect your buyer to pull out of you don’t leave the kitchen/bathroom/insulation in place for instance.