The AA tell me (and I have this in writing) that a car supplied with a spare wheel is illegal to drive without one. They said it would fail an MOT on this basis. They would therefore refuse to attend if a car originally supplied with a spare wheel had no serviceable spare (in practice I'm sure that would only apply to a puncture - they're not going to ask about spare wheels if you're stuck with a flat battery).
I spoke with Kwikfit about the MOT issue - they said that lack of a spare tyre was not a reason to fail the test. They were not, however, sure about whether the car was illegal to drive without a spare.
The AA are talking out of their a*se. Whether or not a car was originally supplied with a spare is irrelevant and, contrary to what I have always believed, it will still pass an MoT if the spare is flat or illegal (see the MoT testers manual here http://www.motuk.co.uk/manual_410.htm
I actually had first hand experience of this last week. Out H&S manager at work has decided that the highly skilled engineers they employ are not trained, skilled or qualified to change a wheel so all our company vehicles have had the spares removed. I got a flat last Wednesday evening which, by the time I realised all was not well and stopped, had caused the sidewall to start to delaminate. No chance of blowing it up so the AA were summoned. I was collected on a recovery truck and taken home. The next morning an AA man arrived at my house, removed the wheel and took it to the tyre fitters for a new tyre and refitted it for me. All a bit pathetic really when I could have had the wheel changed in under half an hour, instead I had to wait the best part of 2 hours to be collected and a further hour the following day while a new tyre was being fitted.
The recovery driver told me that a lot of companies have taken the same view and have either removed the spare wheel or left the spare and removed the jack and wheelbrace (at least then the AA man can fit it). So, considering a huge proportion of vehicles on the road are company owned or leased and most companies have a H&S manager that doesn't actually want anyone to do anything in case they hurt themselves, the odd extra one not carrying a spare because there's a gas tank in the hole is pretty irrelevant.