This morning, I received a certificate from the Environment Agency, confirming my registration as a waste carrier.
This means that I can legally collect waste oil from restaurants, as long as I keep a waste transfer note for each collection, and I dispose of the waste responsibly (e.g. by fuelling my car).
The registration process took 2 months.

I had ordered a plastic card, which didn't arrive with the certificate. For some reason, they put my mobile phone number on the certificate, in the section for Telex numbers. That was a bit unexpected, especially as I only gave them the number on the section of their form headed "Please give us your contact details in case we have questions about your application". These are minor points, but generally the Environment Agency were very helpful on the phone and processed the application in the expected time.

I'm off to my local kebab shop for some oil...