I would go for BD's Diamond Club and get the bmi/MBNA Mastercard, for which you get 20,000 miles for simply obtaining and using the card.

Regardless of whether you fly them much as Nick says the Cash+Miles redemption is very hard to beat, and with a 20,000 mile head start you're on you're way. The one thing that is of concern is if the lunatics at Donnington Hall decide to mess around with Diamond Club the same way they have with the airline itself.