I was just looking at the how the American IRS determines if you are in fact an employee versus an independent contractor. Among the factors listed that would lead to the determination of being an employee and not an independent contractor:
Behavioral Control-
When and where to work (e.g. Monthly Roster)
What work must be performed by a specified person (e.g. Captain, First Officer duties)
Training for the job
Financial-
Reimbursed expenses (e.g. Per Diem)
Exclusive contract (e.g. cannot do work for another company at the same time)
Regular guaranteed pay rate
Type Of Relationship-
Paid vacations
Sick leave
Disability insurance
The contractors work is presented to customers as its own work (e.g. the pilots are Norwegian Air Shuttle pilots)
I know this is based on the American IRS standards, so obviously they do not apply to the UK, but interesting to note that among the many factors listed, several of them would apply to contract pilots.