How many Kw is your old elec shower?
If its low powered and/or the head is scaled up, you should consider a higher Kw shower. These can heat more volume of water as it passes through the shower, so you get more flow.
Go for a 10 kw, a good make, eg Triton, Mira. You may have to change the power cable though. Any shower larger than 8 Kw needs a 10mm supply cable all the way back to the consumer unit.
If its low powered and/or the head is scaled up, you should consider a higher Kw shower. These can heat more volume of water as it passes through the shower, so you get more flow.
Go for a 10 kw, a good make, eg Triton, Mira. You may have to change the power cable though. Any shower larger than 8 Kw needs a 10mm supply cable all the way back to the consumer unit.