you are very unlikely to find many places advertising rooms on the major booking sites that far ahead. Some hotels have yet to decide their room rates for May 2023. The Accor group (Ibis, Mercure etc) often don't work much more than 6-12 months ahead, sometimes less as I posted recently re. the Ibis Budget in St Omer.
I am part of a group that sometimes has >50 people attending a weekend event over the first May bank holiday weekend. Fortunately, others have organised this weekend. My trips have needed a max of 9 rooms which I usually book 4-6 months ahead.
For 15 rooms this far ahead, I suggest you go to the Logis hotels and Brit Hotels website, identify suitable hotels that are in the required area and have the number of rooms you need. Then contact the hotels direct to check if the rooms are available on your dates. The hotels are all independently owned, using the brands just for marketing. For the large group mentioned above, there is usually a deposit required when booking, but the organisers have always negotiated a half board rate at a favourable rate. Final balances are also requested in advance usually. Often if x rooms are booked a free room or two might be granted. The organiser has nabbed the first one and any additional "free" rooms are prizes in a raffle.
The likes of booking.com often charge 15% commission so you will get much better rates going direct, as long as you are willing to stump up the deposit and pay the balance in advance. . Inevitably, this far ahead, people will drop out so you might want to have that conversation about losing a deposit or the drop-outee finds a replacement.
Good luck.
