Why did the new-look 2010 Tardis have a St. John's Ambulance logo on it?

It is a return to an earlier incarnation of the Tardis. There is a lot of discussion as to why it has happened on chatboards (links not provided, as the ones I scanned contained content that was not appropriate for this site, as well as relevant suggestions). General consensus seems to be that we'll find out eventually if it has any significance.

http://www.themindrobber.co.uk/tardis-police-box.html

As above, it is a copy of the earlier tardis. The significance on the earlier tardis would mearly have been to blend in with surroundings. As the police recieved basic first aid training from St Johns Ambulance and police boxes contained first aid kits, I think that the cross would have been a fixture on most police boxes in the 50s and 60s.

https://www.pinterest.co.uk/pin/436215913885486106/

https://www.flickr.com/photos/31363949@N02/14215124416/