Yeah, my boss used to go ballistic when I put any white in the tanks of the ones I drove. Aerial 17m up to a Wumag 260 also on a MAN chassis 
The fuel card links that were offered before is what I used as well as visiting local depots of the company around the place, when I could. Otherwise we had to external containers but that is a [zb] way to do it 
Good Luck with your quest
Anywhere in london to get it as often had to use white to keep the fridge going
the bp on the A10 at Ely sells cherry
If you’re coming out of West Wales, Nant-y-Caws filling station about 2 miles east of Carmarthen on the A48 would be handy before you hit the M4.
There are quite a few indies round here who do red, I’ll update as I remember them.
There is a garage by bala lake in wales that does red. South mimms does red too.