You need to speak to the Chf Clk, who will know this as it is bread and butter stuff, however, the detailed rules are contained in ASPIs (Air Secs Personnel Staff Instructions) and can be found in the RAF Pubs Library through DII etc. You can be promoted and offererd Further Service, but there are limitations. ASPIs state:
FURTHER SERVICE WITH REDUCED MES
37. Further service is not normally permitted for those with an MES below A4G2Z1. However, in those ranks/trades, in which there are particular manning difficulties, personnel may be considered for further service. The list of rank/trades which are critically undermanned will be identified by PMA 7a (in consultation with PMA 6/SM1B (HQPTC).
38. The Board will take into account relevant training, experience and performance as well as the latest medical advice. If the Board considers that FS would be appropriate, the normal period offered will be 3 years; this offer will not constitute any form of precedent or guarantee of continued service thereafter. Second and subsequent FS reviews will be dictated by the manning requirement at the time of the review.
39. Personnel offered FS in accordance with the above paragraphs will remain eligible for discharge under QR 607(10)(f)(ii).
Hope this helps