Mains flow at the kitchen cold tap is ~20 litres/min - the hot tap is
the same[2].

Heatrae Sadia recommend an absolute minimum 20lpm flow from your mains for
the Megaflo. This is definitely on the low side of what you should be using.
You are certainly limited by the maximum flow from the mains, rather than
the system inside your house. As a very rough guide for the maximum flow
rates for hot water you should expect for mains pressure systems:

Combi boiler (non-storage): 10 lpm
Heat bank: 40 lpm
Unvented cylinder: 75lpm

Obviously, these will vary a lot between manufacturer and boiler power etc.
However, none of these systems can provide more flow than your mains supply.
If you only have 20lpm from the mains and it is not practical to upgrade the
supply, then a combi might be more appropriate than an unvented cylinder
(being cheaper and smaller).

well but the shower flow rate is only around 9l/min.

Shower hoses and heads are notorious for slowing down the water. In
particular, if they are designed for combi boilers, they will do their best
to reduce the flow below 10 lpm for more reliable operation. It is possible
to get wider bore hoses and bigger holes in the shower head if your system
can provide a greater flow. If the shower head was designed for an electric
shower, it would have an even slower rate.

Christian.