Some branches certainly used to. If in doubt, turn up with a sharp hand saw, a couple of pieces of 3 x 2 or CLS and a pair of folding trestles and do your cutting on the car park. I've done that before to get bigger lengths in my small van
I have to say that I agree with Foxhole. In all probability your cupboard will need to be rectangular, but any worktops, etc would need to be scribed to the walls. I've generally found it easier to cut-down standard cabinets to fit into awkward spaces. The carcass can be assembled using black carcass screws on the trimmed side and you can generally arrange things so that the trimmed side (where the screw heads are visible) is sandwiched against the side of the next unit along. A bigger problem is cutting down the doors IMHO - because that's only possible with a very limited range of designs.
Let's face it, B&Q are never going to give you an accurate cut - if I ever need to use them I aim to get my timber cut 5 to 10mm oversize (so stuff will go in the car or the van - I don't have a big van, so) and I then trim to the required size myself using a fine tooth blade on a circular saw (this can be done using a fresh fine tooth hardpoint saw as well - it's just somewhat slower) and do any cleaning up with a sharp block plane.
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