Language classes, French radio and television will help in your quest to settle here. My school-French was pretty rusty, so I went to language classes when I first arrived. We listen to French radio in the car and watch the actualités (news) on French TV – though we do turn over to British TV for Downton! Your 'O' level French might need brushing up: language does change over the years, and what you learned at school is not necessarily the French you need today.
Many of the French will speak a little English, and shop staff often go out of their way to help foreigners. The local tax office might even have someone who speaks English to help you 'arrange your affairs', and the EDF electricity board have an English-speaking helpline. However, this isn't enough to make you truly happy in your new environment. You must mix with French people whenever you can. Perhaps you can join the 'Comité des Fêtes' in your village. Help out in your community as often as possible; it will be noted and appreciated – and your French will improve enormously.
CULTURE
The culture in France is subtly different to UK culture. The French have a different way of doing things: they think differently and have different values; they prioritise differently and live quite differently. For example, you have to get used to the long lunch hours, when banks and shops close.