All hospital consultants are employed under the same contract so earn the same basic NHS salary.
However, certain specialties tend to do better in the private sector than others e.g. orthopaedic surgery, urology, ophthalmology, dermatology.
Such specialties involve relatively quick procedures which can be carried out on a generally fit patient population with relatively minimal risk in terms of indemnity and getting sued. Which is why spending a Saturday doing back-to-back cataract operations or mole removals can be quite lucrative.
There is a lot more to it than just that, but that's the simple answer to your question.