Do you have any kind of picnic bag or coolbox, and some ice blocks to go in it? (Or even buy a bag of ice, or have 1 meal frozen to thaw in the box but keeping it cool at the same time).
Do you have any plastic plates and cutlery and glasses? If not, buying disposable ones like for parties would do for a first trip. But I use Ikea kids ones, as they are plastic, different colours (so DD and I can tell who owns which one), and the plates have a decent lip so can act as shallow bowls as well.
A torch - you probably will have some lying around at home for powercuts etc.
Some kind of cooking appliance to heat water and a pot with a lid/kettle for that. (Pot is handy because you could opt for something else cooked too, but don't feel you need to).
Sleeping mats for UNDER the sleeping bags, as that's where most of the cold comes from.
If you have any rugs, picnic blankets, regular blankets as well (and car space), have some spare with you.
Pillows, for comfort, from regular beds.
Hot water bottles taken from winter storage spots, and a woolly hat for wearing in bed (and woolly socks in bed too).
Do you have any folding chairs for the garden, or a folding table? You can live without them if you don't but it would make it easier.
Bringing some spare rope and tent pegs is sensible, and cheap, a mallet (even a regular hammer from your toolbox would be ok for a trial run) would probably be useful after all this dry spell as the ground will be harder. (If you have spare rope, you can use that with 2 sticks or tied between trees to make a washing line for towels/togs etc to dry out).
Regular clothes, toiletries, towels etc. Shoes - a pair that are easy to get on for late-night treks to the toilet block are useful as well (crocs, wellies, flipflops etc). Spare SOCKS.
Bring a tea towel for washing up. If you have any kind of basin or bucket, that would be useful for washing up/moving things to washup areas etc. And a small bit of washing up liquid (use a travel sized bottle to bring some from under the sink at home as you don't need a lot). And a couple of black sacks - rubbish bags, and another for wet/dirty clothes for coming home.
The only thing I think you NEED to have/buy are sleeping mats and a torch.
And while I'd suggest mostly buying meals rather than cooking, you might want to bring a chopping board and small sharp knife from the kitchen to be able to make a sandwich, cut cheese, etc.