I asked Hornby, producer of Airfix models, when they will market a 1:6 model of the WDM20; I got this reply:
"Dear Mr Muller
Thank you for your e-mail suggestion. Although we currently have no plans to produce this kit this year, we do keep a record of all customer requests and product suggestions, which we do take into consideration when planning all future ranges.
Yours Sincerely
Stephanie Brown
Customer Care"
That's a clear message; bombard Stephanie with requests for such model
. Address is customercare@hornby.com
email (option): viaconsu # planet dot nl
Hello Hans,
I just sent them an email and placed my order for one.
Maybe if enough of us actually place an order (even though they don't make one) they'll be motivated into producing one! Fingers crossed!!!!
Ok, ask for M20 and 16H then 
email (option): horror@blueyonder.co.uk
Ok guys, did you already send a message to dear Stephanie at Airfix/Hornby, for a good, big size model of our WDM20, and other Brit War bikes?
Let them know we are interested!
Address is customercare@hornby.com
Do it now!
email (option): viaconsu # planet dot nl
Tamaiya makes the midget-size one! But I - and many other with me I suppose - want a 1:6 size M20 (and 16H, and Ariel, and 3HW and ....). And Airfix/Hornby are British 
so they are the first to listen to our wishes
email (option): viaconsu # planet dot nl