If services are being provided on a business footing the business should, if being well-run, aim to build up a buffer to continue paying the staff, be it one worker or many, for a period when there's no custom. It should also be able to cover NI, pensions and other costs - including transport, phones etc where appropriate. It should be paying at least the statutory minimum levels. In order to make this a viable business the rate paid by the client should be larger than the statutory minimum level by some factor.
That factor might depend on the additional facilities required, such as a cycle and phone for a courier, but in principle that factor could be determined for a particular type of service. There's then a very simple test to apply: you pay less than that, you've got an employee and you handle PAYE, NI, accept that you are responsible for employment rights etc.
There's no reason why the gig economy terms shouldn't be available for businesses that require that flexibility of labour but it should be clearly recognised that the gig worker is taking on the business risk that the engager wants to avoid but should be paid accordingly and taxed as a business.