Erm, if your willing to pay in advance and seeing as your going to be regularly flying the same route, you can buy a book of BMI commuter tickets. These give you instant silver and instant lounge access. BMI call it "Routepass".
You have to buy them in full economy or business class though and you do get the option of changing your route later if you need to. The minimum number of flights you can book are 10 but you have to book each as a one-way. Since it's route specific I am going to assume this means a book of tickets will get you 5 return flights on your route, but you'd best check this with BMI first.
Price is meant to be a saving on the usual full-fare economy prices, but then will still more expensive then cheapest of cheap.
Originally Posted by BMI
By purchasing a routepass, the individual traveller automatically qualifies for silver card membership.
Note that the page witht he information goes on to mention 8000 miles but this is for the *company* routepass rather than the individual.
Full information is at:
http://www.flybmi.com/bmi/en-gb/plan...routepass.aspx