It can be difficult to change Pound Sterling Travellers Cheques at any of the major banks in Fes, and even a 5* Riad may not accept them, so if you are planning to travel to Morocco you should bank on cash ( large denomination) preferably euros or Visa/MCard.

The dirham cannot normally be obtained outside Morocco due to currency restrictions.   It may be possible to obtain a limited amount at your airport of departure but most Forex Bureaux run out quickly. Ensure a small supply of Euros which you should change up at your point of entry to help you pay for transfers and incidentials until you can get to a bank.  Banks offer the best rqte of exchange which may be up to 20% better than you can get at your hotel or on the street.   Most shops will accept euros but don`t expect a good rate of exchange.