We have a similiar dilemma in the Pacific Northwest. On cold rainy days, guests will reasonably want a warm house in the evening.
You can lock your thermostat, but on rainy days give them access?
As for your hot water heater, you will need to educate your guests.
In America, I have never seen your water heating setup. We can't really save electricity by turning off the hot water heater. To manually turn-off the hot water would not be natural or easy to me.
So a timer would be your best way to go, I guess.
Eventually, you will love a remote control smart themostat and electrical valve/timer. They pay for themselves by giving you control, documenting usage and reducing your stress.