..how many airports/airfields are available in the uk and ireland that could facilitate scheduled air services but at the moment do not?
stab3.5up I agree that this is all about maximising potential for scheduled air services. There are plenty of existing runways all over the UK and Ireland, but whether or not they are in the right place is, as you say, down to the economic pros and cons.
I'd split them into four categories of potential:
Intercontinental (eg widebody to NYC or DXB),
European (eg 737 to Canaries/Cyprus),
Regional (eg E90 to Spain/Poland), and
Local (eg ATR to DUB/AMS or domestic).
Intercontinental
MoD Boscombe Down (Wiltshire)
Bristol Filton - sadly recently closed - once had OLT service to Toulouse
RAF Brize Norton (Oxfordshire)
Bruntingthorpe (Leicestershire)
RAF Coningsby (Lincolnshire)
RAF Fairford (Gloucestershire)
Farnborough (Hampshire)
RAF Honington (Norfolk)
RAF Kinloss (Scotland)
RAF Lakenheath (Suffolk)
RAF Leeming (Yorkshire)
RAF Leuchars (Scotland)
Llanbedr (West Wales)
RAF Lossiemouth (Scotland)
RAF Lyneham - due to close
RAF Marham (Norfolk)
RAF Mildenhall (Suffolk)
RAF Scampton (Lincolnshire)
RAF Waddington (Lincolnshire)
Warton (Preston)
Wattisham (Suffolk)
RAF Wittering (Lincolnshire)
Woodford (Cheshire) - sadly recently closed
RAF Wyton (Cambridgeshire)
RNAS Yeovilton (Somerset)
European
Baldonnel (Dublin)
RAF Barkston Heath (Lincolnshire)
Cambridge
Carlisle
Chalgrove (Oxfordshire)
RM Chivenor (Devon)
RAF Church Fenton (Yorkshire)
Coventry
RAF Cranwell (Lincolnshire)
RNAS Culdrose (Cornwall)
RAF Dishforth (Yorkshire)
Hawarden (nr. Chester) - used to have OLT Toulouse service
Kemble (Gloucestershire)
RAF Linton-on-Ouse (Yorkshire)
RAF Odiham (Hampshire)
North Weald (Essex)
Regional
RAF Benson (Oxfordshire)
Biggin Hill
Cranfield (Bedfordshire)
Gamston (Nottinghamshire)
Haverfordwest
Lasham (Hampshire)
RNAS Merryfield (Somerset)
RAF Mona (Anglesey)
RAF Northolt
RAF St Athan (South Wales)
RAF Syerston (Newark)
RAF Topcliffe (Yorkshire)
RAF Woodvale (Merseyside)
Local
Blackbushe (Hampshire)
Duxford (Cambridgeshire)
Enniskillen (Northern Ireland)
Lydd (Kent)
Oban
Oxford - scheduled services starting soon, March I think, to IOM and JER
Swansea
Yeovil (Somerset)