Not very impressive but I can think of these ones in the Southern/South Western region:
North Camp 4 down to 2 (Guildford-bound island platform still stands, with a fence where the trackbed was, likewise the Reading-facing blocks still exist on the opposite platform)
Farnborough Main 6 down to 2 (original island on what are now the fast lines, the London facing bay and the town side bay for the military/town trains)
I believe Winchfield and Brookwood had island platforms at one point too.
Havant 3 down to 2
was Southampton Central's 5th (bay) platform ever used for passenger services?
Portsmouth Harbour 5 down to 4
Salisbury 6 down to 4
many stations along the West of England line which are now on the single line, still have platform faces on the opposite side (such as Crewkerne)