I believe that, under the old rules, you could borrow up to £8,500, however, anything over £5k was liable to tax. The rules on eligibility are dependant on your Service (21yrs+ for RN and 35yrs+ for RAF/Army). If you have in excess of 10yrs to serve, you van delay repayment and if you leave the Service before repayment and arrears is taken from your gratuity.
I believe that LSAP was reviewed, even if only to ensure that the rules for eligibility became Tri-Service, but I am not aware of any increase in the ammount that you can be given.
I have heard, however, that if you are a teacher in London you may be entitled to a loan of up to £100k to buy a house!
kipper