I was actually aware (learned that in the 80's) that the UK used atmospheric/unpressurized water heaters. Though I might expect a calorifier
in apartment buildings, etc.
Agree that a "properly" sized accumulator/expansion tank would be acceptable if there were no PRV's, check valves or mixing valves in the system. If you look at the manufacturers installation
instructions, you will fine that sometimes they indicate a separate expansion tank on the discharge of the water heater.
This is the case with the Solaris /SureJust units the OP was looking at BTW. Both come with a mixing valve on the hot water discharge, which requires a second expansion tank on the hot water side of the unit, per the manufacturers instructions.
I guess my short answer would be to simply read the installation
instructions. The working pressure limitations of the UK units might catch a few US boaters by surprise. Knowing how much guys like reading directions and all. That really was all I was trying to say.