have you already got a combi? What's the water flow from the mains like? As you have two bathrooms that will be very important.
The Danfoss RAS C2 and C3 are very neat valves and IMO the best, I prefer them to the Drayton, so that's a good sign.
The more powerful boiler will be able to deliver more hot water to the taps, but this is only useful if you have a big incoming supply. An older house may have a small supply pipe, in which case you would need to dig it up asnd lay a new one all the way to the road to get enough. Otherwise if you try to use two or more taps at a time you might just get a weedy trickle. If you have a lead or old steel pipe it would be advisable to replace it anyway.
The installer should have measured the water flow at the kitchen sink or garden tap.
Flow is not the same as pressure.
The flue from a big combi will throw out a lot of steam in frosty weather, so have it positioned or extended so the plume will not blow past a window or (preferably) the front of the house.
BTW it is very important to have the condensate pipe plumbed to a nearby internal drain (this should be easy in a kitchen or utility room) and not run outside, where sooner or later it will freeze, and your boiler will stop working just when you need it most.