The strange thing is that TOTAL seem to be charging almost the same for 91UL as for 100LL. I expect they are hoping to push down 100LL volumes and once enough airfields stop stocking it, they can then screw the market.
At the Total booth, they told me that there are currently two companies in Europe producing AVGAS: Total in France and Shell in Holland (they meet once a month for breakfast to agree on the price). Production in the UK stopped some time ago. In Total's terminology, 91UL is AVGAS. The number 91 is MON (motor octane number) versus the RON (researched octane number) common for car fuel which tends to be higher. The 100 octane fuels that Shell and others try to sell across Europe only have a MON of 88.
So 91UL is similar to 100LL and therefore about as costly/inexpensive to produce as 100LL.