How about a couple of the washable bedmats for childrens beds? They are nicer and more comfortable than waterproof mattress covers, and take away the worry about a child wetting the bed.
A washer-dryer is very useful, as you can't rely on the British weather, and that makes it difficult to hang washing out - especially if you are going out for the day and won't be there to take it in if it rains.
A good selection of brochures for local attractions - but not a disorganised heap that includes loads of duplications and out-iof-date pamphlets (I once spent a whole evening at a cottage sorting out their cupboard full of leaflets! That was after sweeping the kitchen floor because it was so dusty, and sending dh out to buy loo roll as there was none in any of the bathrooms/loos!
A cafetiere is vital for me, as is a good kitchen knife, paring knife and a vegetable peeler.
One lovely cottage we rented had a cream tea ready and waiting for us - the table set, teapot ready by the kettle and scones, butter, jam and cream in the fridge.
A list of local babysitters would be wonderful too - we have, on one occasion, gone out for a meal leaving the dses with a babysitter we'd found through a local nursery, but usually there's no way of finding someone to babysit, and it was such a treat to go out just us.
Beetroot - I'm shocked at your stories - I wouldn't dream of leaving a cottage in such a mess - I try to leave it as tidy as I'd like to find it, with the kitchen cleaned up and the washing up either done and put away, or the dishwasher running, at least. And I almost always strip the beds and fold the sheets, blankets etc, and pile the washing in the corner of my room. The only time I didn't do this was when the instructions for the cottage included a very stroppy one telling me to strip the beds. I'm afraid I got stroppy and decided not to!