Do you mean that you wish to break your journey in central London?
If you do, my advice would be to book the relevant trains from London Euston to Manchester Piccadilly, and make sure you are there in sufficient time. You then have a choice, you may obtain Oyster cards for everyone and have your railcard discount added to them (assuming it's not a Two Together or Family railcard) for an Oyster fare on a Southern service to London Victoria, or a train to some other London station. The Oyster single on all trains but the 'Gatwick Express' route is £5.30 after railcard discount.
Can anyone say if Gatwick Airport can process Oyster transactions to get the railcard discount added? Is it possible to buy a new Oyster card there? If you do not wish to use Oyster you can opt for a Gatwick Airport to London St Pancras Anytime Day Single, route Not Underground for £6.80 with railcard. You may then break your journey without question at London Bridge or Blackfriars, Farringdon, City Thameslink or go all the way to St Pancras. You could also travel via Clapham Junction to London Waterloo, from where you may use the walking interchange to Blackfriars. Validity at other London Terminals is likely, but may be somewhat disputed locally.