I have recently had a megaflow system fitted but because of very
low mains water pressure the output from taps and showers is very
poor. Is there a solution to this problem?

Check that all stopcocks are working and fully open. Then compare with next
door to see if there is a supply problem in your neighbourhood.

If it is good, consider replacing your old incoming water main with 25mm
MDPE.

If it is bad, you may need to install a large roof tank and a single
impellor pump to pressurise it.

Quite frankly, your plumber should have checked pressure and flow of your
supply before recommending a mains pressure solution.

Christian.