LPG in 47 Kg bottles is VERY expensive, Just filled 2 of mine (used for the gas hob so each one lasts nearly 2 years, so long you forget to check until BOTH have run out. oops) and that was £150. I would not be using it for hot water or space heating. We only use it for coking because nothing beats a gas hob.
An electric boiler on E10 actually makes sense, you can pretty well heat a well insulated house real time just by heating in the off peak times with no need to use any peak rate electricity for heating.
If you are going to be moving 47Kg bottles about, get a gas bottle trolley.
I still favour heating the DHW tank to about 45 degrees with the ashp and using an in line modulating instant water heater to raise it to final temperature if there has been insufficient solar pv to do so already with the immersion heater.