I'd say no, they do not increase the prices. But it will be very hard to get the cheap seats due to competition.
What I have seen is that the prices start at the same low level when the sales start.
There is however a small number of seats for that lowest price and at popular times many people will try to get those seats.
Some times I have been lucky buying in the middle of the night, on the day they became available, other times I found at 1 AM the seats not yet on sale and at 3 AM the cheapest seats already sold in the train I needed.
If you can be a little flexible, like taking an other train on the same day, you might be able to still get the cheapest seats.
Otherwise you may be willing to get with the 'one or two tiers up' which are more expensive but still way less than the full price.
Best avoid the first day of the holidays in France and all other countries involved in your travels. Better look into traveling on Thursday before busy weekends or on the Tuesday after.
If there is a specialized travel agency for train travel near you, you might talk with them before you book your tickets. I am not sure but I think some companies might be able to reserve train tickets before they actually go online for everybody to buy.
An option might be to book your train to Paris as soon as it becomes available, with Eurostar that might well before the French trains are, and plan a couple of days in or near Paris. You can then buy separate tickets in France on the best time of the day rather than being restricted by the few you can use when traveling from London (or elsewere in England) in one go.
Note, you will pay more for a stay in Paris than you would pay extra on your tickets buying them full price.
Check out when the trains are comming up for sale, as in the summer holidays they might start at a different time from the norm. There are a few questions on this site that might have helpful answers for you.
This one which asks how early the reservation in France can be made and maybe this one, which asks whether you can put train tickets or reservations in France on hold while waiting for additional tickets to come available.