When you pass the written exams, why not apply for an A/G Certificate of Competency. It's another string to your bow, it's free, and it means if you're considered suitable, the holder of the airport radio licence can allow you to operate solo using A/G phraseology before you validate as a FISO, plus they're unlikely to expect you to take on that responsibility without paying you. The only drawback is you can't count A/G time towards the MER for FISO.
An airfield which is published in the AIP as being FIS can't just revert to A/G without issuing a NOTAM to that effect. Some airfields are published as being able to offer both services, however I understand CAA SRG are trying to get away from this and ensure a consistent level of service is porvided.