Apologies @syvvs not had chance to reply any earlier than today! So, long story short, with my expedite request, i was supposed to be registered on the AIS portal and scheduling the interview myself as soon as CEAC showed ready, however, neither the NVC or the Embassy instructed me to do so!
I waited around 4 weeks for the Embassy to schedule as the letter from the NVC i received by expedite approval suggested thats what would happen, and when id received nothing just on the off chance I emailed the visa unit at the London Embassy to establish how much longer I would be waiting considering it was supposed to be expedited. It took them 5 days to respond but they eventually did and told me i needed to schedule myself; as you can imagine, I wasnt a happy bunny, id waited 5 weeks and missed out on ample interview opportunities (the earliest one that was showing once I noticed my CEAC status had changed to ready was 14th August).
The 30th September had nothing to do with visa availability, it was more that it was just the next available date for interview on the AIS portal (i missed out by a day on an earlier one on the 16th Sept)!
Medicals are booked two weeks before, and not entirely sure about visa availability as @appleblossom has eluded to, but given my interview is the last day of Sept, i dont anticipate there being much of a wait for a visa to be issued post interview either way (being the refresh date is 1st October).