I am in the same situation as you (see my post) and have looked at all the above brands you mention. Apparently WB are not fully Opentherm compatible and Vaillant need an aditional component (VR33 i think) to work with opentherm thermostats (Nest, evohome etc). But with VR33 the warranty goes void.
Fully Opentherm compatible brands that I have considered are Ideal, Intergas and Atag. I've read on Atag and they're big in the Netherlands (where opentherm is mandatory) - and in the UK they are well known in the commercial sector but relatively new in domestic market. I have also read good reviews of the Ideal vogue.
Fully Opentherm compatible brands that I have considered are Ideal, Intergas and Atag. I've read on Atag and they're big in the Netherlands (where opentherm is mandatory) - and in the UK they are well known in the commercial sector but relatively new in domestic market. I have also read good reviews of the Ideal vogue.