I haven't seen a kit, but have bought everything separately. We have been driving to France for the past few summers so have most things.
The AA website is usually good for ensuring you have everything list to download
The headlamp converters are very easy to fit. They come in a pack of two - and there is a fold out leaflet which shows the position for each model of car. If you have a newer, top spec car you may not need them. DH has a new BMW - he can adjust the angles mechanically, so no stickers necessary. (however he didn't realise that he would have to re-set the angles each time he started up the car.. oops!) i advise reading the leaflet and working out where they are going before you get there. My car still has glue residue on the headlamps, so i Know precisely where I am sticking them. (usually do it whilst on the Tunnel).
Vitals are:
Hi-Vis vests. Must be kept inside the car. NOT in the boot. i.e. if you had toget out of the car at the side of the motorway you would be already wearing it.
Warning Triangle
First Aid kit
Spare bulbs (although I have NO idea how to fit them)
Breathalysers -pack of 2 is about £4
The headlamp convertors are a couple of quid
GB sticker
I might have forgotten some things, so check the AA list.
You MUST disable speed camera detection on all your devices which have mapping/sat nav. We have sat nav on all phones, tablets and in the car - so have to remember to ensure that they are off. You can get into big trouble if stopped by the police and they can see that you have speed radar detectors on.
Keep change for tolls. Or get one of the toll automated tags. We have one and it has been great for the past three years. Saves time queuing at toll booths. Try APRR - it's the one I got ours from.
Umm.. anything else just ask.