Why not put in a mains pressure tank *you won't need expension tanks?

Not with a solid fuel appliance - these must have an unpressurised
circuit, as there's no way to shut them down once fired up. They must
also rely on gravity circulation (in case of a power cut).

Caveat - I've never fitted a solid fuel rayburn - only a modern
pressure-jet oil burner. However solid fuel was one of the options I
considered and I had a good think abut how I'd route it in my own
place, and nosed about at other installations.

For your f&e, your routing sounds ok to me. But the main boiler to hw
tank in 28mm - I'd take great care to ensure you follow the Rayburn
installation instructions exactly, to ensure that you have excellent
gravity circulation. Both pipes slope upwards all the way, minimum
number of bends, heat-leak radiator etc. Otherwise you may for
evermore be plagued by kettling/knocking in your boiler.