From what I've seen a lot of greenhouses are very modular...two end
sections, and 2ft middle sections that add to get the right length
(4ft, 6ft, 8ft etc).
The long rails that hold it all together seem to be just straight
strips of metal with no fancy bits at the end, and everything just
bolts together with extra brackets for the corners.

I don't want to spend £300 on a new 6ft x 4ft greenhouse (not after
blowing my money on a new mower), and secondhand ones go for over £100
if you can find them at all.
6ft x 8ft are quite common and can go for around £50....so would there
be any problem in simply sawing this in half to make 6ft x 4ft?....all
I would do is saw the long rails in half and drill new fixing holes. I
would also lose two of the middle 2ft sections. Seems simple enough.

I know this is a bit of lateral thinking, but would it work?
Do better quality greenhouses like Halls have a different construction
to cheaper B&Q ones?

From what I've seen it may work but be careful of oddities such as windows
that may result in the sections being slightly different to one another.
Also, you will probably find that the long sections that you are going to
cut are actually 8ft + a bit to allow for the frames. You will need to be
careful to allow for the frames etc. but presumably if you assemble a single
2 ft stretch (end plus first set of central struts) and then double the
distance from end to centre of strut (2 x (2ft + bit)), you will probably
OK.

Don't call me if you cut too short though ;-).

Paul DS.