Assuming that the mains pressure and flow is good, I'd do it the other
way
round. Have the shower (and possibly kitchen tap) off the combi, and run
the
bath from the cylinder, possibly using the pump.

Eh?!

Showers need pressure as much as they need flow. This means that an
excellent shower can be obtained by a combi, which uses mains pressure but
is limited in flow rate. It will also be less noisy, as no pump is needed.

The cylinder is capable of supplying much higher flow rates. However,
gravity feed might not be able to get sufficient flow rate at the taps, for
which a pump can be beneficial.

Christian.