Are both flights on Saudia? If they are then you should be fine. Just tell them to check any baggage through at the domestic checkin, and get your international boarding pass. Then you'll only have to go to Terminal 2 and go through immigration. If your domestic flight is more or less on time then you should be fine.
If the UK flight is on a foreign airline then you should probably allow longer. You may not be able to check you bag through and you may have to check in again for the international flight. Also, it's a longer walk from Terminal 3 to Terminal 1.