If you do need 11 travel days in Europe, you do have no choice. It will always be a two month validity when buying a flexi pass. 

It wouldn't be possible to cover 37 days of travel with a one month pass anyway. That's more than one month.

You can buy the flexi pass 10 days in 2 months and then buy a normal ticket for one of the travel days with the shortest travel.

10 days / 2 months - £296

15 days / 2 months - £366

What is your shortest/ cheapest travel day? As long as it will cost you less than £70, you should buy the 10 day pass.