To begin with an answer, it should be reminded that the socioeconomic living conditions in XVIIIth century France had a sharp divide between major cities (and in particular Paris) and in the countryside (where they had only marginally evolved since the late Middle Ages). If you have seen Dangerous Liaisons by Stephen Frears it is something depicted quite accurately in the depictions of the countryside stay of Valmont.

In the countryside, as late as the second half of the XIXth century, consumption of beef was rare, as meat consumption in general was very limited, and mostly pork and poultry raised in each home (see L'alimentation paysanne en France entre 1850 et 1936, Rolande Bonnain-Moerdijk, Études rurales Année 1975 58 pp. 29-49). Where I am from (rural central France) most farms still raise (and kill, before Christmas for the pigs) their own chicken and pigs for domestic consumption. Consumption of beef was very limited as cattle was either (1) used to pull farm equipment or (2) a source of dairy, in particular to make cheese. As such, killing an instrument of production for meat would have been limited to old oxen or cows no longer able to fulfill their role in the farm. This can be seen in the fact that most traditional French dishes with beef include a (long) boiling/stewing to make the meat (from old cows/oxen) softer and more palatable (we could mention the pot au feu, the boeuf en daube, etc.). In more affluent farmsteads, there was also some consumption of veal on celebrations, as veals were a byproduct of dairy farming (the famous Veau gras) : see the numerous recipes such as the blanquette de veau, the tête de veau, etc. Finally, some measure of illegal hunting (rabbits in particular) probably took place, complementing the protein intake of the rural population (also in the same article).

In the cities, there was already a significant consumption of beef in the early to mid XVIIIth century (see Boucherie et hygiène à Paris au XVIIIe siècle, Sydney WATTS, https://www.cairn.info/revue-d-histoire-moderne-et-contemporaine-2004-3-page-79.htm). As you probably know, Paris had (during the Ancien Régime, and until 1943 (!)) a toll system for various products at city boundaries (the octroi, whose gates still subsist in parts of town) : the archives of this fiscal service (which were quite well kept : our fiscal civil service has a longstanding tradition of efficience) are useful to know what kind of products the Parisians ate in the XVIIIth century. In the 1780s, the abovementioned articles quotes a figure of 70 000 cattle heads for about 650 000 inhabitants, leading to a consumption probably close to London on a per inhabitant basis.

The difference would have been in modes of consumption : beef was eaten in almost integrality (and it remained the case in France relatively late : my grandparents were relatively fond of mou (lungs) and had eaten fraise (stomachs), and from personal experience, coeur farci (stuffed heart) and foie de génisse (young cow liver) are quite tasty. From my understanding, british customs had a much more early emphasis on red meat and its direct preparation (flame broiling / roasting) as opposed to a general French custom of stewing/boiling.

Historians (S. Watts, Meat Matters :The Butchers of Eighteenth-Century Paris,Ph. D.Dissertation,Cornell University, 1999.) concluded that the XVIIIth century saw a generalization of beef meat consumption in the emerging urban bourgeois classes (as game from hunting was reserved to the nobility), driving down the price of lower cuts and non-muscle parts for the lower classes. This rise in interest in beef dishes can be seen in cook books of the time such as La Cuisinière bourgeoise (first published in 1746), giving rise to (1) the traditional French way of cutting a beef carcass (which greatly differs from the US style) and (2) most of modern-day traditional French cooking (which takes its roots in this transition from court habits and appropriation by the bourgeois class in major cities, with some input from local traditions).

There was (and still is) a tightly knit corporation of butchers in Paris with its own traditions and secret speech (Louchébem), one of its key traditions was the "défilé du boeuf gras" (fat ox parade) which was its own carnival event (and was all the rage in the late XIXth century).

So :

• Beef consumption was not high in the countryside.
• It was definitely significant in major cities and someone like Alexandre Dumas would have already been familiar with beef
• Nonetheless, it was not cooked "in the English style" that is as pan-cooked or grilled beefsteaks : his surprise would have been in this practice (he would have been familiar with beef stews or other preparations).