Oil storage regulations - start here
http://www.defra.gov.uk/environment/water/quality/oilstore/pdf/oil_store.pdf

I can't imagine any marina setting aside a special tank for one customer. Installation costs and insurance would be more thanb the increased cost of red.

So why not do it yourself ? Buy, or make up a bowser. Bunded fuel tank on a trailer, or buy a real jobby from Witham

http://www.witham-sv.com/infopage.php?ID=46&Overide=0

and invest in a few thousand litres with some like minded friends.

A difficulty will be finding somewhere which would allow you to refuel with your own tanker, unless you can find a private mooring ?