There are so many articles on the net giving very good reasons for not using anti-freeze as well as not using deionised water, and actually the cooling water is in the ‘inner’ glass tube, not the outer.
I am also in the UK. I still use the traditional reservoir and pump as the cooling system and my laser is located outside in a shed. For winter and generally in colder weather I use a 50W fish tank heater to stop the reservoir freezing. I don’t leave my pump running but I to leave a 60W tube heater on which is laid in the back of the machine under the tube position.
I use Distilled water which I buy from apcpure through ebay, about £25 for 25L and free delivery. There are many suppliers to order from on-line. I do not use any additives and have never had a problem with bloom of algae. My reservoir does have a lid on it which helps to keep the water clean and helps stop evaporation.
I mentioned having the fish tank heater on in cooler weather as well. The reason is I find a cold tube of below say 15C doesn’t perform as well and my power settings need to be increased to compensate. Keeping my tube above 15C is so much better.