This is generally applicable to UK registered cars driving on the roads of continental Europe, the light conversion notes will also relate to cars registered in the Republic of Ireland as these are also RHD and traffic predominantly keeps left there as per the UK.
So I had to make 2 emergency trips to Europe, my first trip took me briefly through France and the Netherlands, then a longer period of time in both Belgium and Germany, I had already done my research on what I needed to take with me and what I had to do to adapt my car for use out there, so I purchased a Halfords Motoring abroad kit, which came with a first aid kit, a warning triangle a 'GB' sticker so as to identify my car's country of registration, and a pair of 'Eurolites' headlamp beam converters, which I replaced on my second trip as the removal of these is somewhat destructive.
Mandatory in France is breathalysers in the car, Halfords sold these in packs of two and the packaging stated the French police used them, though these will probably never get used I will keep them in the car anyway.
I also purchased a stick-on speed conversion chart as all speeds in Europe are in KPH whereas in the UK it's MPH, though my speedometer is calibrated in both units the KPH reading is actually quite small, another sticker for my windscreen was a pictogram of which side of the road to drive on in Europe, the sticker being reversible for tourists to the UK.
France's motorway network is also heavily tolled, the sign 'Péage' means toll, the tolls work by on entry you get a ticket from the first tollbooth, which is on the left hand side so in a British car it can be quite a chew on, then drive on until you exit the motorway and when you do, you will come to another set of toll booths, where you can pay cash, card, or using an electronic tag called 'telepéage' (an orange 't' is above those lanes in which you can use the tag if you have one), for me the toll between Calais and Paris Charles-de-Gaulle airport was approximately £20 each way when converted from Euros
Insurance wise your fully comprehensive insurance here will give you third party only out there, however I was able to have equal cover in Europe for less than £5 a month more, you don't need to make special arrangements with your insurer while the UK is still part of the EU, your existing UK photocard driving licence is also valid while the UK is in the EU, no international driving permit is needed while you drive abroad just yet.
Your headlights must be adapted, to do this you will need 'Eurolites' headlight converters that stick onto a prescribed area of the headlight, what this does is reduces light going off to the left from your headlights (left-hand traffic configured cars have the headlight emit more light to the left on dipped beam), these must remain attached to your headlights at all times in Europe (I fitted mine on both occasions once I was on Dover port) so as not to dazzle motorists coming the other way, and must be fitted day and night.
The AA have a comprehensive list of things you'd need to travel on European roads and not fall fould of the law, including information on low emission zones and their names in the country or countries you visit, they also offer breakdown service if you are abroad and your car so unfortunately happens to break down out there.
And finally, fuel, the RON of Unleaded fuels in the continent is similar to the UK (95/98), however the cheapest fuels available are E10, that is they contain no more than 10% Ethanol, in the UK E5 blends are starting to appear, the Ethanol content should not be cause for concern, Toyota recommended for my car that the blend not exceed E10, check your car owners manual before you leave, some places the fuel is a bit cheaper than the UK but others it is not, also pre-pay pumps can authorise up to £100 however you will get back the difference and only pay for the fuel you actually have bought, and it may take a few days in which to return to your account, I cannot comment on Diesel fuels as I have a petrol car and therefore have no need to buy Diesel fuel.
And remember, keep right in continental Europe.