I only get 12-13 litres per minutes at those taps. I have have both taps
on at the same time the flow drops off. :-(

Not good. The only hope is that the behaviour was caused by a partially open
stopcock or similar restriction.

You can still put a mains pressure system in if you genuinely intend to
upgrade your supply at some point, but until then, it would be a downgrade
from a gravity based system. If you did decide to install one, consider
feeding the heatbank from the cold water cistern. This could then be
connected to mains instead when the supply pipe was upgraded. If you have no
intention of upgrading the water supply, then a standard gravity system will
be much cheaper.

Christian.