I tried this the day before yesterday; went to the post office with my V5C which states first registration at Aug 1980. The PO system, which apparently accesses DVLA direct, would not give the option to give tax exemption for historic status. Even though I had, as a precaution, taken my heritage certificate as evidence, which shows a build date of June1979, the post office said there was nothing they could do about it.
I phoned the DVLA who said "that's strange" and told me to send in my V5C and original heritage certificate, which I did yesterday. I guess wait and see what happens.
thats the PO not knowing what to do, fill in the change of taxation class section to state 'historic' the PO then sends it of and issues you with a receipt saying zero paid its that simple.
the DVLA will show as still taxable because if hadn't been changed on the V5.