You can add anything you want to a TA (although don’t ask me how you do that technically with an OR tenancy agreement). How about, “always keep an inflated balloon in each room”? The issue is, when it comes down to it, will a judge consider it a reasonable clause to have inserted into a contract.
The bigger question is what are you reasonably going to do if the tenants do not comply? Will you really pursue a S8 for non-compliance? How are you going to prove that they breached this? Do every viewing with a thermometer? What if one part of the house is at 9 degrees?
If my tenancy agreement said keep heating on a 10 degrees I would have done so from day 1, if I have heating on I turn it up full the living room heats up well, I just sit in a pair of Bermuda shorts yes I have vintage Bermuda shorts. the kitchen has a crappy little wall heater it’s pointless putting it on but I don’t need it, kitchen does not feel cold, bathroom had another silly little wall heater I do use it if I’m sitting on the lavy at night but don’t need it on any other time. Bedroom even with heating on I don’t think it even hits 10 degrees unless I add 2 portable heaters, in the room you end up in a cold sweat.
I think this is an unenforceable clause & not sure keeping a house at 10C will necessarily stop damp & mould, which I assume is your intention. Perhaps instead include an information sheet that explains about cold damp, condensation & mould.
Alternatively ensure the affordability criteria you use shows the tenant can realistically afford to rent & heat the property - no one would choose to live in a house that cold!
I’m assuming the reason for wanting the tenants to keep heating at minimum of 10 degrees is to prevent burst pipes in winter - which cause a terrible mess and possible damage, and house will be freezing whilst it’s sorted. Which is normal, sensible - and a good idea to make sure your tenants know. Could be added to agreement/contract along with the reason why.
If you want to have any hope of enforcing a self-drafted term, then it needs to be very carefully worded, preferably by someone who understands the law.