Many travel companies use screen-scraping technology and cannot replicate raw entries required to perform the correct search.
Main 'city' codes in the UK are
LON (covering 5 airports - LGW,LHR,LCY,STN.LTN)
MAN
LBA
BHX
EDI
GLA
NCL
BRS
The use of the radius feature (when possible) will open up a great number of possibilities and will display charters and lowcost airlines
eg
BRS with a radius of 50 miles will open up CWL (Cardiff) and EXT (Exeter)
BHX with a radius of 50 miles will open up Coventry (CVT) and Nottingham East Midlands (EMA)
LBA (50 miles) will open up Manchester, Doncaster Sheffield (DSA)
MAN (50 miles) will open up Liverpool (LPL) and Blackpool
You get the idea.... (I could be here for hours
)