Post Office Ltd will not "Exchange" these notes.
They can be accepted, as payment towards a bill or as a deposit into a Bank Account, but due to restrictions on Cash Holdings, Post Offices cannot exchange them.
And I wouldn't attempt to just buy a stamp with one as again it'll probably be refused!
I'm actually surprised by the banks agreeing to exchange them, as everything I've seen used the wording "Accept", not "Exchange" and in our neck of the woods they try and palm off to us!
To be honest the only ones I know who will exchange are the Bank Of England.