Leasing would do away with the hideous cash pit that is QQ
It isn't mandated that QQ is used, just that independent assessment is made to verify main contractor safety claims and provide independent advice. There is no reason why MoD Flight Test Aircrew can't operate in partnership with any other UK defence contractor to get the same (or better) results, quicker, and therefore for overall less money.
I am clearly ignoring the fact that the MoD will never buy or lease this aircraft of course, but the point is valid for any other aircraft procurement or modification programme!