I currently have a gainsbrough 10.8kw electric shower, but I have
always been unhappy about its outflow. Is there a way to boost the
cold water pressure to the shower or perhaps there is an off the shelf
electric " boosted " shower out there.

I believe that there are electrically heated power showers available.
Alternatively even the smallest single impellor shower pump will work,
provided that you have a gravity tank feed available. You must not pump
mains cold water.

Of course, if your problem is that the electric shower can't get a decent
enough flow rate when on full heat, then this won't be solved by a pump.

Christian.