I contacted Bridgestone over here and they just said they weren't approved in Europe and so unavailable - they didnt explain why.
The Boulevard is known as the Intruder M1800RZ over here and has a 240/40R18 tyre which is obviously no good for the Rocket. Maybe that's the only size Bridgestone have got approval for?
Edit:
I just remembered a similar thread on the .com site and the likely reason for non-approval in the EU is due to the Max being a 240/55 making the o.d. too big. The EU regs demand a tolerance of +/- 3% on tyre diameter, the 240/55 is outside that tolerance.