Nationally:

https://www.facebook.com/groups/holidaycottages/ Holiday Cottages UK is the group where I have received the most bookings total (caveat to say I used to be admin there and there may be some skewing of my visibility there as a result)

https://www.facebook.com/groups/627924374521622/ Last Minute UK Staycation Cottages & Lodges

https://www.facebook.com/groups/PetFriendlyHolidayAccommodationUK Pet friendly

https://www.facebook.com/groups/lastminutepetdogfriendlycottageshottub for those with a hot tub and dogs

Lodges To Rent with Hot Tubs for hot tubs

Dog Friendly self catering accommodation IN UK best group for dogs

https://www.facebook.com/groups/482262575456983 Farmers Wives Holiday Accommodation (no need to be a F’s wife, just rural)

https://www.facebook.com/groups/2350832361782228 New Farming Wives Holiday Accommodation

https://www.facebook.com/groups/7023544997705702/ UK Staycations: UK Holiday Homes & Short Term Lets

https://www.facebook.com/groups/727200081069539/ WI unofficial

https://www.facebook.com/groups/456202251881074/ solo with dogs

Locally: (there are oodles more to choose from; these just give an idea of what to look for)

https://www.facebook.com/groups/237383441819554 West Wales Holiday Lets

https://www.facebook.com/groups/3563938966990912/ We’ve got room for you - Pembrokeshire

https://www.facebook.com/groups/lakedistrictaccommodation Lake District Accommodation

https://www.facebook.com/groups/cornwalanddevonholidays/ Devon and Cornwall (there’s the same in other areas, but this one is very active)

https://www.facebook.com/groups/accommodationindorset/ Only Dorset but good as an illustration - fewer than 1000 members but I've had many bookings from here because it's specific and small. People are already hot leads when they have found the group and there's a much smaller pool of respondents.

Scotland:

Holiday Homes Scotland (one post per month but you are allowed to reply.

Lodges with Hot Tub in Scotland