Your pension is treated as income and effectively treated as if it were taxed (unless you have the funny pension of course, which is treated completely free of tax). But it is done so by abatement of your tax code - a moot point perhaps, but it does mean that it isn't tax free in the first place, so therefore isn't liable to any further erosion.
The military pension is an unfunded scheme which is based on 'prerogative instruments'. PIs are not subject to approval, annulment or amendment by Parliament - they derive their authority directly from HM and has a kitty which is drawn directly from the 'Consolidated Fund', so the amount would seem safe. The annual amount required fopr each year is voted in in Parliament and it'd be a brave person who dissents.
Under the Naval and Marine Pay and Pensions Act 1865, PIs for the RN and RM are by an Order of Council, for the Army it is the Pensions Warrant 1977 and for the RAF it is the QRs (for the Royal Air Force). Of course, the rules can change for new entrants.