I do not know about UK winters, but in my case i had a water pipe bursting open at -12°C.. It was an old pipe in the kitched ceiling i actualy never knew it was there, so it wasn't even realy outside the house. I made a new plasterboard ceiling and screwed it to the old wooden boards. Used kit to seal off the edges along side the walls. Now the old pipe in the ceiling did no longer get warm air from the house as it did from the old ceiling gaps. The first winter at minus 12 it cracked, luckely i was at home when it did and saw the water running down through my newly constructed plasterboard ceiling. 🙄 I can't imagine what would have happened if i wasn't at home at that time and it would have leaked for hours..
Quite risky to keep the pipes outside under pressure all winter like that, wont have much structural damage in the garden, but if you are not at home you can expect a rather scary bill from the water company.. 🙂
But if it worked like that for the last 30 years, you must have rather warm winters.
Good luck..