If it is ill advised to put the whole lot in the loft - which bits are a
good idea to put up there?

No problem. I have a similar size house to you. 3 bed, 1 bath.

I have a Worcester Bosch Greenstar 28HE in the loft feeding a DPS Pandora
heat bank, also in the loft. No additional cold tanks at all. Hot water is
mains pressure, with a flow rate of 40 litres per minute @ 60C.

Basically, both a system like this and a combi are going to provide
absolutely superb showers, as they both provide mains pressure and the
shower head or valve will limit flow rate to below the capacity of the
boiler.

The main differences between the systems will be on bath filling performance
or on having multiple showers. If you have/do neither regularly, then the
combi will save you money.

Note that if you install in the loft, there may be additional costs, most of
which didn't apply to me because they were already present.

1. Loft must be boarded.
2. Loft must have lighting.
3. Loft must have mounted loft ladder (can fold away).
4. Hatch must have guard rails.
5. You need a slightly more expensive vertical flue.
6. You need longer pipe and electrical control runs.

Christian.