Before you travel
As Britain is not in the Schengen zone, you will need a valid passport to enter France, unless you are a national of another European Community country and have an approved identity card, or are very good at talking to frontier police. If you plan to drive you must have a valid driving licence; if you come by car current UK car insurance is valid in France, but it is useful to ask your insurance company to give you a European accident declaration form, just in case.
French electricity points are not the same format as in the UK, and to avoid frustration with electric razors and hair-driers, think to bring an adaptor plug (available on cross-channel ferries as well as in most port-side shops). If you are staying in a hotel, adaptors may be available.

If you are on medication, remember to bring your prescription, and even if not, bear in mind that rules about medication sold over the counter in French chemists are not necessarily the same as on the UK, so you may not be able to buy some products without a doctor's prescription.
Many people in the hotellery and restaurant industry in France speak some English, but if like many others, you never mastered French at school, it is always a good idea to bring a pocket phrase-book or dictionary - it makes a holiday more fun to communicate, even hesitantly, with the people in the country you are visiting.

Driving in Normandy
You already know, of course, that it is advisable to drive on the right on French roads (including Normandy), but it's easy to forget when you leave the port or take a roundabout for the first time, which can be embarrassing at best, disastrous at worst. Most roundabouts in France are now "à l'anglaise", which means that priority is given to cars already on the roundabout, as in the UK. Elsewhere, "priority on the right" is still the rule, except on trunk roads, unless there is a "stop" or "slow" sign to modify that, and it is especially important to bear this in mind, particularly in towns, where failing to do so may lead to a short sharp lesson in colloquial French.

Staying in Normandy 
Contrary to popular belief on the other side of the Channel, the French, and especially the people of Normandy, do not usually wear berets, ride bicycles sporting strings of onions, and eat garlic all the time. This may disappoint first-time visitors, but they will find consolation in the warmth and hospitality of the local people, some of whom may indeed wear berets, and all of whom will appreciate your interest in their region and your attempts, however clumsy, to speak their language.  The French are accustomed to hearing a variety of foreign accents in their daily lives, and they genuinely appreciate the visitor's stumbling attempts to communicate in the "langue de Molière", particularly if their guest's own mother tongue is the "langue de  Shakespeare."    

Do's and don'ts when in Normandy
1. If you wish to try out your French - and you will be made even more welcome if you do - don't use "tu" until it is used towards you (a notable exception : if a policeman uses "tu", you're already in trouble - don't make it worse by returning his familiarity).
2. Don't tell your taxi-driver that you've never been to Rouen before.
3. Ask for milk in your coffee at dinner if you must (but if you wish to impress the waiter at the bistro), ask for a "noisette" and he will bring you a small black coffee with a little jug of milk.
4. Do ask the wine-waiter's advice before ordering : that's what he's there for.
5. Do leave the main roads and order the dish of the day in village restaurants.
6. Do try to travel outside key weekends (Easter, beginning of August, New Year) : these are the target dates for lightning strikes by public-sector employees, and you may find yourself stranded at an airport or railway station with thousands of other unfortunates.
7. If you do nothing else while in Normandy, DO go to Mont Saint Michel !

Bienvenue en Normandie !