I have confused your thread sorry. You were asking if an unvented cylinder could be placed after the boiler. This is the normal configuration. And Evohome will work for you.

What I was saying is that, if you had solar, wanted a combi but felt the combi was not enough for simultaneous showers then an unvented could be placed before the combi. This unvented would be preheated by solar.
The combi then potentially could get a 60C inlet, and has to do very little work to top up the heat. So could still be sufficient