Can you identify this freehub from a 2014 Boardman CX Comp?
I made a guess. it turned out correct:
Trek/Bontrager (Select) Freehub Body 561948 (repl W268886)
Available from Pedalon (where I ordered from) or GB Cycles (from whose catalogue I figured it out).
GB Cycles listed a few similar alternatives but they can be discounted:

561949 is 11-speed (although worryingly listed a 130mm OLD, whereas my bike is 135mm OLD - but there are no sizes on the 561948)
W268886 has been superseded by the 561948
W211339 has sealed cartridge bearings

I'm still waiting for Boardman to come up with an actual part number or Halfords reference, although it's a moot point now.
I've only actually done a quick test fitting tonight, but the freehub fits the existing hub and the cassette fits on the freehub, so I think I'm good.
Not a direct answer, but I called up Boardman. Not entirely surprised they didn't have a part number, but they were able to offer up a reference with which I could order from Halfords. They have me the usual runaround, but hopefully should have something that fits middle of next week. In the meantime I guess I'll continue getting fat.

Boardman's reference on Halford's system: 341740
Receipt description: &quot;FHB CB/Car/VD Splin F th&quot;
Cost: £19.99

Except it turns out that doesn't fit. Different gender. Drat.