You're throwing 13.5 GBP - or more than 16 euro - at it per 100km.
That's a lot .
At the very best, expect somewhere between 6 and 7 L/100km - that's 40 to 47 mpg (Imp) , 33 to 39 mpg(US)
Check out these fuel records:
Overview: Austin - Mini - Spritmonitor.de
and
Overview: Mini - Mini Classic - Spritmonitor.de