Haven't had a look at prices but it may work out easier just to buy a 1-6 off-peak travelcard (or use Oyster if you have one).
Coventry - London Zones 1-6
Route Any Permitted
Off-Peak Day Travelcard
£31.30
is indeed cheaper than
Coventry - Kingston
Route +Any Permitted
Off-Peak Return
£31.60
however the former is not an eligible fare for the railcard concession on Virgin either.
Nevertheless the travelcard is valid on the 1031 Virgin service to London Euston, arriving 1132, if that is not too late.
Strictly speaking only an Off-Peak Return to London Terminals (£28.80) is eligible for the concession with Virgin under revised rules. How consistently this is enforced varies.
I advise getting the London Terminals ticket and pay for the journey between Euston and Kingston separately using Oyster PAYG. If you load your railcard onto Oyster (instructions
here), you get 1/3 off Off-Peak fares, which between Euston (Underground) and Kingston is £2.85 each way when changing between Victoria Line and South West Trains at Vauxhall. The fare is £3.05 each way if interchanging at Waterloo.