I can think of a couple examples as follows:
Acton Main Line - Platform 2 - This platform used to get one train a day only. This was the last Eastbound Heathrow Connect service of the day. But i have not seen anything use it for years now.
Guildford - Platform 7 - This platform shares a track with Platform 6 and is therefore not used. The doors are opened on the other side. But the platform is not blocked off and could technically be used if they wanted to.
Southall - Platform 0 - This is a through platform and i see no reason why it could not be used. But i have never seen it used before. I believe it is not even electrified so i suppose it is less of a use now then it was a few years ago when almost everything was diesel operated.
Tonbridge - Platform 4 - This is very little used but does actually have an early morning service and a late evening service booked. But it is certainly not a well used platform. It would be easy to think it was never used.
West Drayton - Platform 5 - This is a through platform for the slow lines and can technically be used by any trains. But it is very rare to see anything call at this platform. I have not seen it happen for a long time.
Worthing - Platform 1 - This is a perfectly usable through platform. However nothing is booked to use it. There are no trains timetabled through this platform. It can still see occasional use during disruption but it is very rare these days to see it get used.
There are also dozens of stations with four platforms on a four track layout where the platforms on the fast lines very rarely get used.
Some examples include:
Balham, Battersea Park, Bushey, Cricklewood, Ealing Broadway, Earlsfield, Elstree & Borehamwood, Finsbury Park, Harrow & Wealdstone, Hayes & Harlington, Hendon, Horley, Iver, Kentish Town, Langley, Mill Hill Broadway, New Barnet, New Cross Gate, New Southgate, Norbury, Oakleigh Park, Purley, Purley Oaks, Radlett, Selhurst, South Croydon, Southall, Streatham Common, Taplow, Thornton Heath, Vauxhall, Wandsworth Common, Wembley Central, West Byfleet, West Drayton, West Hampstead Thameslink, Wimbledon, and probably dozens and dozens of others.
However all of these platforms do still see fairly common use during engineering works and disruptions and late evenings etc so they are not as little used as you may think.