No such allowance for a forces rail card
From the armed forces railcard webpage:
5. Will I have to pay the full fare if I forget to bring my Railcard when I travel?
Yes. You must show your valid Railcard and Photocard when you buy your ticket. You must also have your Railcard and Photocard with you on each journey and present to staff when your ticket is checked aboard the train. Failure to present both your Railcard and Photocard will mean a full fare is payable, which is not refundable.
And also the terms and conditions:
Conditions of issue and use of reduced priced tickets ('discounted tickets') bought with the HM Forces Railcard
1. Tickets for your journey should be purchased before boarding the train. You must present your Railcard and Photocard whenever you purchase a discounted ticket. You must have your Railcard (and Photocard) with you on your journey and always show the valid Railcard and ticket to rail staff at their request.
If you do not show both the Railcard and the ticket, you will be required to pay the full price Standard Single fare for your journey as if no ticket was purchased before starting the journey. In some cases a penalty fare may be payable. This does not apply if there was no ticket office at the station at which you began your journey or if the ticket office was closed and there was no ticket machine from which you could buy a discounted ticket.
2. Minimum fares and time restrictions apply as detailed here.
3. In the case of some low price tickets the actual discount with a Railcard may be slightly lower than that advertised.
4. The right is reserved to revise these Conditions and availability of the tickets mentioned here. The Train Companies will endeavour to give three months' notice of any changes before they are due to come into effect.
5. Tickets issued to the holder of the Railcard are issued subject to the National Rail Conditions of Carriage and the Conditions listed here. Copies of the National Rail Conditions of Carriage are available from any staffed station ticket office or online at
www.nationalrail.co.uk/times_fares/nrcc/