A FISO needs to be trained at the airfield that they are working at. This is a simple process and can be done in a short period of time. The costs are small and the costs for 3 or 4 FISOs to man the tower would be far far smaller that full ATC.
Lots of airfield have FISOs and handle Business Jets, and they would be able to communicate with Birmingham for Radar Clearances for Airways Departures.