It’s not common practice but that’s what I did.
Mine was just the Vaillant boiler. I priced up my buying the boiler versus a quote from a Vaillant installer installing it, with me supplying and installing and setting up, an entirely new set of controls. I found my installer was willing to accomodate me either way, but the difference in cost, plus the slight extra risk [1] simply made it not worth my while. [1] Any after install issues, might well be labour at my cost.