The RHS career structure is such that you have to start from SO(2 years) then to FO(2 years) and finally SFO. Worth noting that the grades of FO and SFO are automatically on 75% and only if there is requirement you will be offered a 100% position according to seniority. So from SO to FO there is little or no payrise.
If enough people turn down the 'stupid' contracts easy offer them, only then easy will be forced to hire RHS position on appropriate grade and pay.