It's close to Gill Sans Display Bold but the centre of the M is different

Keep Calm  Suggested by rocamaco 


Keep Calm And Type On  Suggested by marijnkampf 

Alternatively you could use the Keep Calm and Type On font from http://www.keepcalmstudio.com/font. It has both the crown and the upside down crown included. I've specifically designed it to match the font from the original Keep Calm poster.