Can anyone recommend a boiler that is suitable for fitting in a loft
space? We (will) have 4 bedrooms and two bathrooms (both with baths
and showers) so the boiler must handle that.

Almost any modern boiler that you can squeeze through the loft hatch will be
fine. The loft is a particularly good place for a modern condensing boiler
as the plume is clearly not a nuisance going through the roof. Also, modern
boilers tend to be lighter than their older cast iron counterparts.

Whether you go for combi or lots of different storage based solutions
depends on your intended usage profile. However, with 2 proper bathrooms, a
non-storage instantaneous combi would probably not be suitable. You should
be looking for either a large storage based "combi" (which could be
difficult to get into a loft), or a two box solution. In either case, you'll
need to check external dimensions against your loft hatch before ordering.

Using a heat bank or unvented cylinder (rather than a gravity fed cylinder)
would be advantageous, as you don't have to have a high up cold water
cistern, or combination cylinder that may be prone to running out if two
baths or showers are run simultaneously. It also allows a shower in the loft
should you later convert.

Provided your mains water supply has a good flow rate, these solutions will
provide excellent shower and bath performance. If fitting yourself, a heat
bank will be simpler to fit. If fitting professionally, an unvented cylinder
is more likely to be understood by the fitter. The advantages and
disadvantages of each has been done to death in the past, as a quick google
will show.

There are requirements for there to be a proper boarded floor, a fixed (but
retractable if desired) loft ladder, guarding, electric light etc. These are
all useful to have anyway, but must be budgeted for if not already present.

Christian.