Hi Paul,
JS Harris on here posted a heat loss calculator on eBuild, I don't know if it is here, but from what you say your house could well be approaching passive house levels of insulation, it depends on your air tightness, the insulation levels sound pretty high You'll barely need any heating at all.
The plumber is talking absolute nonsense on the two boilers. A lot of trades just aren't used to the amount of heating required in a modern well insulated house, you might only need 5kw of heat input if I had to guess. I am building a house twice the size of yours with a pool and will only have one 40kw boiler.
As Mike say, once your heat loss gets really low, actually how fast you want to heat your water starts to drive the size of your boiler. Your UVC will probably be ale to take 30kw of heat input and I would think this is the size of boiler you need. Anything else is overkill.
We have a 250l UVC at the moment and never run out of hot water, except when people stay. There are three of us. If you have people stay a lot and lots of people try to get up at 5am to catch a flight or something like that, running out of hot water is a pain. The real killer is if someone has lots of baths, a 150l bath fill plus two showers could drain a 200l tank.
I would say a 300l UVC is the right size for the house you describe. Is the suggestion of 2x200l so you can turn off one sometimes? 300l will have slightly higher standing losses than 200l, but the cost of an extra cylinder and extra installation costs would dwarf this. Indeed a 300l cylinder only costs around £100 more than a 200l cylinder and fitting costs are almost as high as the cost of a cylinder. Also two cylinders will take up a lot more room.
In fact, sorry to say this, but the more I think about it I wonder if your plumber is at it, he is suggesting a system with 2 boilers and 2 cylinders which is totally unnecessary and will cost around £4000 more than it should.