You are permitted to drive a French-registered vehicle in the UK for a period of up to six months, as long as your insurance policy covers you for driving in the UK. We would also suggest bringing the vehicle's registration certificate with you so you can prove you are the registered keeper. Headlight deflectors are not compulsory but it is a sensible addition, and if your numberplate has the letter F and the EU symbol on the numberplate then you do not need to add an 'F' sticker. There is no requirement to change the units of the speedometer to miles per hour, but if your car has the facility to do this then it is a sensible option as it will make sticking to speed limits easier.