The property manager handles all contact with the tenants (and ultimately it's not my property either), so I'm not sure exactly what the conversations were. I know they were offered heaters but declined them.
This website would suggest that a kettle is sufficient for washing purposes.
"Lack of hot water is an emergency regardless of the season. Tenants can use a kettle for boiling water for washing purposes, but that doesn’t mean landlords can hold out on the repairs.
Some repairs might take more than the reasonable time, in which case the landlord needs to provide equipment for heating. If your landlord cannot fix the problem in time and cannot provide any replacement heaters, you should ask for temporary accommodation. You can stay with a B&B or a hotel for a few days, while waiting for the repairs to be done. This should be covered by your landlord, but if it is not, you have a strong claim for compensation with the courts."
Yes, an electric shower might be a good idea in the future, if and when this shower breaks, but hopefully now there's a brand new boiler in place there won't be any issues for a number of years to come.
I'd like to think DH is a good landlord. From our point of view at least, the managing agent seems pretty good too (other than charging DH £100 for a new shower head once!) and these tenants have been great too - they've been there for about 5 years now I think. There were concessions during Covid when they couldn't afford to pay their rent and they have a short 2-month rolling tenancy contract at their request as they're on a social housing waiting list.
I do think OP's landlord should offer some compensation though for the disruption that the works will cause.