This is getting crazy! Why would he want to install an instantaneous electric shower, with the necessary hefty cabling, when he already has a stored water shower? And still keep the cylinder full of hot water? For economy, as JohnD said, use E7 overnight, and if it's used up in the day, which is unlikely unless he uses a lot of hot water, top up at normal rate. The cost won't break most people's bank, and the OP didn't suggest economy was his main motive, he just wants to rearrange his bathroom.He already has that, the water stored in the cylinder and heated overnight on E7 at cheap(er) rate. If he can ovoid using it all up showers, by using a more economical electric shower, he will have plenty of hot water. Despite JohnD's negativity, modern decent spec. electric shower are fine.
Right - I'm away for a quick (electric) shower, then a browse round the shops..