ISTR, the CAA rules say something like "One piece of hand luggage not bigger than and heavier than , plus a handbag/purse, or briefcase, or pair of binoculars or camera".
I'd suggest that the laptop is a modern equivalent of the briefcase, probably weighs about the same as a briefcase containing paperwork, and that's how I'd probably play it if someone was being a jobsworth.
That said, I've never had a problem on most airlines with a small roll-aboard (and I mean
small - it's a shame this bag isn't available in the UK) and my laptop bag, except on smaller aircraft, where I'll either check-in the roll-aboard in advance, or offer to gate check-it if the flight is busy.
Cheers,
Mike
P.S. Here's a link to
Easyjet's baggage policy - which explicitly says that a laptop isn't part of the hand luggage allowance, it's in addition to it.