I need a shower pump for the shower, otherwise it just won't work -
the showerhead being higher than the HW tank

The height of the HW tank is totally irrelevent. The only influences are the
height of the water level in the header tank and the bore and effective
length of the pipework. (Effective length includes penalties for elbows and
tees).

It is very useful to have the pump for the bath tap. You are right not to
want it on the basin, though. Can you replumb the basin to before the pump?

(although CW feeder above HW tank is a couple of feet higher, so
may get a dribble - but not enough to have a decent shower).

This is another reason to install bath pumping. On mine, I found the low
head coupled with the double check valves made starting the pump on shower
to be unreliable. However, it was easy to nudge it by blipping the bath tap,
without needing an ugly and expensive "negative head" upgrade for the pump,
which usually consists of yet another decaying green tangle cord from the
ceiling.

I had a system just like you are considering, with mains cold and a single
impellor on the hot side for bath and shower only. However, you might want
the separate cold feed if your mains are that bad and use a twin impellor
pump. You may need to seriously increase the size of your cold water header
tank to ensure you don't run out.

Am I right in thinking that as it's unlikely more than one thing is
going to be using CW at any 1 time in the bathroom then a single feed,
with bits taken off it when needed won't have a big impact on flow, or
do lots of joints and corners seriously slow things up?

One feed will be fine. If pumped, 15mm won't be too much of a disaster and
will even get hot water quicker. I suggest buying a proper pipe bender.
Avoid elbows unless absolutely necessary. Flowed bends are cheaper (if
you've already got the tool), allow greater freedom in choice of angles,
drop much less pressure, allow more flow and are less noisy and prone to
hammer.

Recommendations for a reliable and reasonably powerful shower
pump? (Not done one before)

Everyone recommends Stuart Turner. However, I had a Showerforce single
impellor for 99 quid on the hardest water imaginable. No problems, even
after two years.

Christian.