I was reading Top Gear Magazine issue 120, and there's a piece about people auditioning cars to appear in Gran Turismo 4 and the article states that several cars that didn't appear were going to. Here is the full list
AC (Various: 212 S/C, Superblower, ACE Brooklands)
Bentley Continental '52
Ford Fiesta RS1800 '94
Ford Sierra RS500 '87
Lancia Fulvia & Monte Carlo (Indirectly Mentioned)
Lotus F1 Car (Year not specified, listed as "Classic")
Lotus Elan, Excel (Indirectly Mentioned)
Marcos (Various: Mantula, LM400/500/600, Mantis GT)
MG (Various Pre-1980: MG B, MG A, TD, Midget)
MG Metro 6R4 Group B '85
Morgan Aero 8 GTN GT2 '03
Morris Minor '50
Renault 4 '61
Triumph (Various pre-1980: TR3/4/5/6/7, Spitfire, Stag)
Westfield (Various: Megabusa, SEiGHT, FW400, XTR2)