It looks like the answer is a Worcester Greenstar running vented into a
direct heatbank with a zone valve for the rads and a second zone valve for
the towel rails and airing cupboard rad.

Ensure you get the right model. Most Greenstars require a sealed pressurised
circuit. I believe that the Greenstar HE29 "Conventional" is the model you
need.

Sounds like you want to configure as S-Plan Plus. When designing the layout,
for maximum future versatility, run an unzoned "main" round the house and
put the zone valves where the actual zone circuit starts. This way you can
add new zones easily, rather than having to route a pipe all the way to the
boiler.

Christian.