Originally Posted by
plunet
2. Go to the Bank of England as per this
page, but as you note it seems they are very busy at the moment. Note their requirements carefully.
3. Go to a specific Post Office branch as per this
page to exchange notes, up to £300. Note their ID requirements carefully, and not all branches will provide this service so check the branch finder on the page linked above.
The list of participating post offices is on the BOE page linked in point 2 and is somewhat more legible there