Sory to tell you, but you can not get it. LSA is only for the Regs, even though you was with the regs it does not mak you one. I when to Kenya with the regs and I have had this fight. Sorry to tell you the bad news.
Reserve Forces personnel are entitled to LSA in certain circumstances, JSP 752 covers this:
JSP 752, Edn 4, Chap 5
05.0105. Reserve Forces Personnel. Members of the Reserve Forces, with the exception of non-mobilised Regular Reserves and Cadet Force Adult Volunteers,will be eligible for LSA in line with their Regular counterparts (but see paragraph 05.0107g).(...this para covers Annual Camp etc, which do not qualify)
Entitlement to LSA is dependant on qualification by either former Regular, FTRS or full time mobilised service, or qualification by virtue of TA Service, which normally means 18 months service and having received a Second Year Bounty. Full details can be found in TA Regs.
TA Regs, Chap 7, and Annex K to Chapter 1 - TACOS on Mobilisation:
10. Longer Separated Allowance (LSA)
a. Personnel with single or aggregated periods of regular, FTRS and/or mobilised service amounting to 18 months or more, or TA personnel who have received their second year bounty, will be eligible to claim LSA at the initial rate after a period of 10 or more days away from their normal TA unit or home. After having been eligible for LSA for 300 accumulated days individuals become entitled to LSSA at the middle rate, and after a further 300 days at the higher rate. Personnel are ineligible to claim LSA during periods of leave of 10 or more days (excluding travelling time). (see also paras 7.125-7.126.)
b. Eligibility for the allowance ceases on the day you are processed through the Demobilisation Centre at the end of your tour.
These are the rules, and LSA is available to mobilised FTRS personnel.