When did your water supply last fail - I live in London and never happened to me
Lucky you. I've lived in Metropolitan Line land and, being at the top of a hill had virtually no water flow regularly (in the summer evenings particularly with people watering their plants).
Here in semi rural Bedfordshire I got £50 credit from Anglian Water for the regular(ish) mains pipe bursts in the village, just last year.
That said I like my Oso unvented HW cylinder with an air bubble in it.