Scott,
perfectly OK with HRMC for private individuals to use the 2500 litres of used Cooking Oil/Bio,
(not rebated fuels or used fossil fuel)
even new purchased Veg oil,(tho this is a very gray area,) for 'personal use' without paying duty.
(thats not commercial/business use,Taxi's, Busses,works vans etc, even company cars.)
www.biopowered.co.uk/wiki/Rules_and_RegulationsMaybe your vehicle is just insured as Social, domestic & pleasure & not business use.
Keep your personal log of fuel useage & things go quite easily.
However if you are carrying your tools of the trade in the vehicle when pulled into a roadside VOSA/DVLA/HMRC
then there can be problems for you.
Being dipping for Red will not be a problem, (running a mixture of waste/burst oil/ATF etc will get tests done on the mix)
they will question you on Running Bio
& if you are self employed.
They will ask to see your reciepts for where you buy your regular duty paid fuel,
ask to see your accounts,tax returns etc etc.
Sadly thats an offence that can loose you the vehicle & its contents.
Its a big risk to take.
george