Why not heat the cylinder fully from a ToU tariff such as Octopus Go overnight, via the HP, and then pump any excess during the day into the immersion to hyper-inflate the energy storage capability of the UVC. Nighttime charge to 50oC then reduced by whatever was collected during the day, with the boost from the immersion going as high as 70-80oC. You will get the better CoP with running the HP, but only at the lower temp plus you'll be cycling the HP on and off with the pockets of solar gain, which it will NOT thank you for.
Once you calculate the wear and tear on the HP, PLUS those attributed losse of getting all of the HP components up to temp for each event, same with all of the interconnecting pipework etc, eg before it collectively then delivers premium temp hot water to give the stated recovery times, .......then the immersion route soon becomes a more attractive choice afaic.