I've done some checking, and here's what I found.
I was a bit surprised that I couldn't find anything about this in the IR's standard documentation, nor on sites like Motley Fool. A bit of googling turned up someone in uk.finance saying back in 2000 that he thought the IR regarded the acquisition value as nil in all these cases, which is what I was afraid of. I phoned the IR helpline, and their off-the-cuff answer was along the lines you suggested above -- the market value on the date of issue. But when I mentioned the other alternative they went off in a panic to check, and phoned back to say that I should indeed treat it as a nil acquisition value. How lovely.
Matti