There is a BS EN for wheeled extinguishers - BSEN 1866:2005 (replaces a 1999 version) for Mobile Fire Extinguishers, defined as an "extinguisher that is designed to be transportable and operated by hand that has a total mass of more than 20 kg and which is mounted on wheels"
However strangely it only covers water, water additive, foam & powder extinguishers, not CO2.
As the volume of sales of wheeled extinguishers is far less than portables it probably explains why manufacturers don't bother going for the kitemark (or their national equivalent) as sales don't justify the expense unlike getting the Marine Wheelmark as that affects their ability to sell to a significant part of the mobile extinguisher market (which is mostly split between marine & aviation markets with a small bit of general industry thrown in)
ABS stuff from Express does have a good reputation, as does Express themselves- I suggest any fire protection company take a look at Express, we use them for almost everything now.