Hope this helps..
I flew exec-jets in the early 90s .. on a self emplyed basis and based in the West Midlands.
Within a few months of starting (and after having registered with HM as being self-employed)... I was notified that if all of my income came from one firm (it did!) I could not continue on a self-employed basis.
Further investigations by my accountant confirmed this.
Not that it mattered much because just afterwards I lost my Class 1 medical.
Although the old Inland revenue and Customs and Excise now operate as a single agency (HMRC) I believe that the "single income" criteria still applies.
Good luck anyway. BM