If you use cooking oil, and/or produce biodiesel or any other fuel substitute for use in your vehicle you must contact HMRC in order to 'Make Entry'. This is to record details of where the fuel is made (or used) and to register your activity with us. You will receive information on how to record your fuel use and pay the excise duty due.

and

Important Note: It is illegal to run your car on any fuel substitute without having first paid us the duty.

... there's no mention of any exemptions for less than 20,000 miles/year I'm afraid.

Here are the relevant links:

http://customs.hmrc.gov.uk/...very long url

and

http://customs.hmrc.gov.uk/...other very long url

--
Simon