Train Ticket + Travelcard but not use actual train

Good Morning,

I'm travelling to London today with my 15, 12 and 8 year old sons and will need to use the tube. We have a family and friends railcard which gives us a good discount on train tickets combined with a travelcard for use in London.

Total cost using the train from where we live and tube will be about £30. If I purchased a train ticket/travelcard from a station nearer to London combined with the railcard it would be £10.70, I wouldn't need a travelcard for the 8 year old. So if I purchased this, drove to say Hounslow and parked for free but jumped on the tube without using the train part would this be ok? I can't see that it would be, as I've paid for the travelcard but wanted to confirm. Many Thanks!