I have been driving round in my Jeep for the past 7 years to and from shows with either a .30 cal or .50 cal on the back, always, I hasten to add covered up, only uncovering at the event or in organised convoys etc.
I keep copies with "copy" written on of all the deact certs in my glove compartment. I have never had a problem with the police. I do believe that if your weapon has proof marks you do not have to hold the deact cert as the weapon can be traced back via the marks, if you want a replacement cert, again I believe it is about £35?? however I feel it is good practice to always buy a weapon with a cert.
The same goes with all my deact weapons I use for display including whichever one I display in the windscreen gunrack, again in a sleeve when travelling to and from events.
I also belong to AFRA http://www.afra.org.uk/ for my PLI which is £15 per year.
If you want to buy a denix get yourself along to Malvern http://www.militaryconvention.co.uk/default.htm or Stoneleigh http://www.militariashows.com/ as they sell them there, the normal trader is "stand up Hook up" AKA Dennis Smith
http://standuphookup1941.com/
As mentioned above http://www.wwiireenacting.co.uk/ is a good source for Denix and they are often on there for sale at cheapish prices along with Denix Thompsons etc.
One last word, it is nice looking to have a weapon on the back but the mount does take up some space and makes it cramped in the back for any passengers.
Just my 2 pence worth.
Paul