Two points immediately leap to mind. Firstly if you have a colleague doing a similar role, how are you self-employed? Are you certain you are genuinely self-employed, ie have other clients, have control over your work, can substitute someone else if necessary, etc etc etc?
Second point is why do you need to compare?
If there is a reason to compare, you need to take into account lack of job security, not working all year round, no pension, no paid holiday, no paid sick pay, maternity pay, employer NI contributions etc. But a comparison will never be exact, and will depend on a variety of factors such as industry.
Setting your rates as a self-employed consultant or similar is a commercial decision, based on the value you add, the marketplace you work in. Not on comparisons to employed staff. One rule I've seen though, which may provide a starting point, is to assume that you work 100 days a year as a self-employed person, the rest of the time spend marketing, managing your business, administering the business, and not working due to fluctuating demand.
That may or may not be the case for you.