You can apply online for a replacement. Still £7.00.
You cannot get done for not having a tax disc. You could only be done for not displaying. It can be checked by a traffic officer that the vehicle has a valid tax.
I am sure that if you apply on line for a replacement you will get some kind of confirmation back from DVLA.
If you print this off & keep it with you it would be a very mean traffic officer that booked you.
Also put the details inside your tax disc holder.
What would happen if your tax disc had been stolen and you did not know ? I know that it is perfectly legal to not have a tax disc at all let alone not to display one if you are going to a pre-booked MOT test. Do you need an MOT test anywhere near to your destination? - Only joking.
If you are going on the Afghan Heroes Ride provided you apply for a replacement and do as I suggest above. If you get any fine I personally will pay it for you. Go and enjoy.
After more searching I have just found this:
http://www.direct.gov.uk/en/Motoring/OwningAVehicle/HowToTaxYourVehicle/DG_10021514Displaying your tax discIf you use the electronic vehicle licensing service or tax by post at the end of the month, there’s now an exemption for not displaying a tax disc. This exemption covers the first five working days of the month to allow time for the new disc to arrive in the post. While you are still waiting for your tax disc you will need to display your current tax disc. The exemption only applies if applications are made before the current tax disc or SORN expires.
In my opinion this ought to mean that you are in the clear as you do not have your current disc to display!!
As it is not the police who prosecute (they report you to DVLA for prosecution). I am certain that they would not prosecute under the circumstances:
More often than not this offence is prosecuted by the Department of Transport (DVLA). The police officer submits form CLE 2/6 (or CLE 2/8) direct to the department. If other offences are involved then the police may prosecute on their behalf, but this is now quite rare, because the DVLA can demand an instant penalty without going through the Courts, and this is more practical for them than waiting for proceedings to take place, but note that as from the 1 June 2003 failing to display a vehicle / trade / nil licence is a fixed penalty offence.
Do
NOT accept any Fixed Penalty Notice you do not have to. Ask for it to go to the DVLA for prosecution. It is very unlikely that they would do so.