Although the 650B movement (excuse the pun) is mainly stateside, in the UK we are blessed with a slew of 'compatible' frames.
How so? – Well the main requirement for fitting 650B into a 26" frame is clearance .. as long as your frame has clearance for 2.5" 26" wheels you should be able to (comfortably) fit 2.0-2.35 650B wheels/tyres. And the UK – being the mud-pit that it is has spawned a generation of mud-friendly (thus 650B friendly) bikes.
The new pipedream sirius r853 is a good un' for 650B wheels … as is the Orange p7, The Cotic Bfe, The Surly 1×1 .. and lots more waiting to be brought to light.
Dave