This is a very interesting topic for me - the biggest problem, although not insurmountable, is that the test for self employment varies according to whether one is looking at the employment relationship according to employment law or whether the HMRC tests are being applied.
I've read fairly extensively on this subject (although my core specialities are TUPE and unfair wage deductions, all questions on those topics more than welcome
) and I'm more than half tempted to provide the 500 words or so which would answer the challenge in Ian's post but as with everything else in employment law, it's generally much more about specific facts than law generally so wouldn't be applicable in every case.
I suspect that with the majority of franchises, work is given to an entity, which can be a sole trader, partnership or limited company, which is then free to have that work carried out by any individual. This is known as the substitution clause and is generally sufficient to confer self employment status on either employment law or HMRC principles. In addition, there is likely to be no mutuality of obligation (work doesn't have to be offered or accepted) which would also generally be sufficient although in many cases the lack of mutuality of obligation won't be enough since HMRC could infer a 'zero hours' contract and apply tax responsibility accordingly.