Class F remains available for airspace classification however, under SERA, it should be used for specific purposes. Generally, these lend themselves to airspace that is in transition from uncontrolled to controlled (think aerodromes outside CAS getting busier - but not yet busy enough to justify CAS). The key issue is that the use of Class F should be temporary in nature, pending the submission of an appropriate ACP to establish CAS.
In the UK's case, with Class F Advisory routes having been established for decades, it's reasonable to acknowledge that it's unsustainable for the Airspace Rrgulator to justify that it was temporary, hence the regularisation of this airspace to become either Class G or Class E+TMZ as fisbangwallop says.