It shouldn't be any more complicated than not fitting one.
The fact that you have a pipe branching off to the loft makes things slightly more awkward than usual, but its not a big problem.
I would fit a full bore lever valve below the tee of the expansion pipe then a tee for the pump outlet...
Depending on your flush valve, you may be able to remove the top section of the valve with a small turn. Check the sealing washer is ok and nothing is on the mating face of the bottom section of the flush valve (scratches, dents, debris..etc).
Not sure if its the company your looking for but might be worth a look,
lucky for you the only showroom in the UK is in Didcot!!
http://enexp.vitra.com.tr/
VitrA Didcot Showroom
Park 34, Collett Way Didcot, Oxon ,OX11 7WB
Phone: + 44 1235 750990
Stuart Turner are probably the best on the market for shower pumps,
their monsoon range are good but pricey, they also have a showermate range which are cheaper, but i haven't installed them so can't comment on quality.
you could try one of these it can be adjusted upto 160mm
http://www.mcalpineplumbing.com/traps/chrome-plated-traps/1-188-cp-plastic-adjustable-inlet-bottle-trap.html
You could make a similar thing yourself for a fraction of the cost, you'll need a 1/2"male-15mm compression elbow, a short piece of copper (maybe ask a local plumber for a small offcut) and a funnel.
Or if you have a towel rail just use a funnel and pour it in the top (after draining a bit out...
I haven't fitted a shower enclosure for a few years so can't remember exactly how they are assembled, but i'm pretty sure the rollers come off.
If you look at the roller from the inside of the door I think there is a cover plate that will slide up or down to reveal a screw.