Unfortunately the only way to get a Tesla is to come off the Motability scheme.
Tesla do not give discounts as they can sell every car that they produce so no need.
Perhaps when their German factory starts production they may need to turn to sources other than private buyers.
They do have plans to produce a compact car for less than $20,000 so this could be aa more reasonable proposition to leave Motability and buy direct, especially as Tesla do offer finance deals and leases.