OAP, to an extent the pension is already means tested. Your tax free sum is reduced by your OAP. Your pensions and pay taxed at 20% is also capped so that you pay a marginal tax at 30% before you reach the 40% limit.
Politically they say your pension is not taxed as it was taxed as you paid in whereas your other pensions were tax free going in, so there is a certain logic there.