Ah, well, it's easy to answer, right? Boilerjuice, Butler, Bayford, Watson, or Scottish Fuels. Simple, eh?
Or is it?
Thanks to John Kelleher who alerted us to the fact that many of the oil companies (that includes both domestic heating oil and other industrial oils) are linked together. Basically, one big company owns many of the others, so many are part of the same larger group. There's a less independent "choice" than consumers might have hoped for.
The biggest consolidating agent is GB Oils, although the info on their website is pretty minimal. GB Oils is based in Warrington, and is the largest single oil-distribution company in Great Britain. It is also a division of a massive services conglomerate, the DCC group (based in Dublin, but operating in at least 4 countries: UK, Ireland, Austria and Denmark). Within the British Isles but outside of England and Wales, GB Oils operates as Emo Oil (Ireland, north and south) and Scottish Fuels. GB (Emo) Oils has expanded rapidly in recent years, executing many take-overs of smaller oil distributors.
On the petrol forecourts, they operate as Shell and Texaco.
It's worth reading up on the extensive details of the mergers and acquisitions that the DCC group has been involved with. Energy distribution names include: CPL Petroleum, Southern Counties Fuel, Chevron Ltd., Speed Fuel Oils Ltd., Jackson Fuels Ltd., Bayford Oil (one of their bigger acquisitions), and most recently, Brogan Holdings (Scotland). GB Oils has also acquired Carlton Fuels, Martindales, the Boilerjuice website, and Pearts in Hartlepool.
DCC has a stated ambition of achieving a 20% share of the British oil distribution market. DCC also distributes LPG, under the Flogas brand.
The Office of Fair Trading (OFT) has been monitoring DCC's oil sector moves for a while. Because of competition concerns (or concern about lack thereof) the OFT made GB Oils sell its distribution interests in the Western Isles of Scotland in April 2010. Those concerns arose after GB Oils acquisition of Brogan Holdings.
What's worse, a lot of the seemingly independently price-comparison websites are not that independent at all: See this list here and which website is owned by which company; most of them ultimately owned by DCC. Quite depressing stuff, really, if you wanted a truly independent search engine! Another supposedly impartial fuel search engine that we know about is Fuel Oil Direct: their site is funded by companies paying to list with them, but they are still a DCC company.
At least now we know why so many companies readily agree to price match with their so-called competitors -- they may not really be competitors at all!