Funny, same journey (Brussels - Calais) on Thursday, dry but riding against really strong headwind, some gusts causing me to tuck in behind screen to stop bike moving around so much. Cross the channel, and rain started on the M25 round London.
Interestingly, filled up in Brussels with RON 95 (Lukoil) before leaving, and the average mpg by Calais was showing 44. 3 mpg. Filled up again at Dover with RON 97 (BP), and by London average mpg was showing 54 mpg! I know these figures are not always accurate, but that's quite a difference!