Supersonic flights were still authorised overland up to 1974/75 according the RAE Bedford flying order book of that period. They used the Bedford Levels as a supersonic corridor; presumably it wasn't too heavily inhabited in those days (can't swear to that though).
This was originally to accomodate the BAC 221 (aka FD2 Mk 2) but was retained 'just in case' after that project ended.