Just like in the UK, the culture between companies (and industries) varies so no one size fits all. Same with working hours. French labour laws have historically placed some restrictions but a lot of companies got round that by paying extra days holiday/overtime for staff working longer hours. So you may find in some companies that people do work long hours but they will then take more holiday. Note that when a French public holiday falls on a Thurs, most people will take the bridge ("faire le pont") and be off on the Fri as well.
I studied in Paris, then worked for a French company in the UK for 7 years (language of the company was French for 3 years before it switched to being English). I later moved to another French company based in the UK but after 3 months was transferred to the Paris office where I was the only non-French person. Note that although these were "French companies", they were massive global players in our industry so very multi-cultural (unless you were in Paris where it was utterly French).
Apologies in advance as this is going to be more stereotypical than I would like and there are always exceptions to everything!
If you're UK based, you won't get exposed to be as much of the French way of working obviously compared to if you were working in France. But it helps to be aware of some key things. Family is important (hence minimal working out of office and especially not at the weekends). It used to be that French dads would start work later in the morning as they dropped the kids off at school before coming to work but they would work later as French mums would finish work early to pick the kids up (so the mums started work early and finished early). Massive generalisation but it was the case where I worked.
Food, wine and coffee are important (quality is more important than quantity). Basically quality of life is very important! This will permeate with most dealings you have with French people so think quality! And of course French quality is almost always the best! 
Don't try to argue about this, it isn't worth the drama. And you may come across drama. The French do show their emotions at work so don't be surprised. They can be much shouting at times. My old French boss (in a German company!) once had a tantrum and stood up, hurled his laptop at the floor and stormed out of the office. It happens.
Meeting tend not to start on time. For a 2pm meeting, 2.10pm is the generally accepted start time. Don't book meetings first thing in the morning (due to people doing school drops offs) or over lunchtime (12-2pm). Lunch is only missed when something very urgent and important comes up.
Lunch at your desk eating a sandwich is generally considered disgusting. I was on my own in my French office one lunchtime so popped out and grabbed a sandwich and was happily Googling away back at my desk when I was discovered by my French boss. He was VERY unhappy with my "Anglo-Saxon behaviour". After that, whenever the guys I shared an office with where out at clients at lunchtime, some random French person would appear at my office door at noon to frogmarch me to lunch with them. No more Anglo-Saxon lunches for me lol It was quite sweet really (and a bit exhausting). Get to a restaurant/cafe (staff canteen where possible) and have lunch with your colleagues, following by a coffee (espresso) afterwards. This is what civilised i.e. French people do. As a French colleague once said to me "if you don't have coffee after lunch, how does your body know that lunch is over?". The French take this very seriously (see quality of life above).
Social niceties are important. It is polite to always say good morning and ask how someone is. If you're in a French office, shake hands (or kiss - "faire la bise" if you know them well) when saying bonjour to each person as they arrive. And yes, you do this every single day without fail. The same process also applies when leaving the office so you need to "bonsoir" everyone. So remember your manners at all times!
I actually love working with French people but I appreciate that for some people they can be an acquired taste. Parisians especially. When I moved to Paris with my last role, I shared an office with the deputy head of the division plus 2-3 other people - none of this open plan stuff, it was a little rabbit warren of offices. The deputy head P was a complete arsehole to me. After a few days, P complained to me that my French wasn't good enough. This was bs as he hadn't actually heard me speaking French as I arrived when there was an American colleague visiting so everyone had been speaking English anyway! It gives you an idea of how unreasonable French people can be if they don't know you. So don't be surprised! But if you can win them over, they are the most wonderful, loyal, lovely colleagues/friends you can ever have! It took me a while and I had to prove my worth constantly - not just with the quality of my work but with how I ate lunch, what I ate and in what order, how I took my coffee, how I greeted people....basically how French I was! Years later, I still get tested by my former French colleagues e.g. invited for dinner at their houses and I get asked to guess what wine we're drinking etc. I have finally received the ultimate accolade from them which is "Hundred, she could be French". 
I try to educate/torture them by introducing them to English food and wine (despite the faces pulled). I took some Nyetimber sparkling wine, which they did drink with the comment "hmm, it's better than bad French champagne" (that's a compliment btw). I also took a cheeseboard of small producer artisan English cheeses (unpasteurised of course - after I'd previously had a 3 hour lecture from a colleague about how the EU is destroying good cheese with pasteurisation). The feedback was "this cheese smells so bad, it could be French" and it all got eaten.
If you show an interest in the French way of doing things / culture / language, it will be very well received.
Sorry for the essay!!!