I don't think 24kw boiler is too small likely about right, however best do a proper heat loss calculation.
Also for efficiency try and find a boiler with good modulation.
Could stick with smaller tank and have a high gain tank installed instead, should heat the water in half the time with a standard cylinder. Look at gledhill, mcdonald and joule, these brands are just as good (sometimes better depending on likes and dislikes) as the megaflow but cheaper, a high gain 300ltr MacDonald is £300 cheaper than the 170ltr megaflow
Theintergasshop is very good. You don't have to purchase an intergas boiler (though I highly recommend one and have one fitted myself) but the guy who owns the shop is very well informed and connected and doesn't mind calling out firms for shoddy product and practices. He used to really promote Honeywell and the evohome range but recently held them to account for their lack of support, poor product availability, excessive price increases and lack of development of the system. Though he still sells the product mostly for existing customers and fundamentally likes it, he's now advocating the Drayton wiser system as what is happening with that is the complete opposite to evohome, they're developing it, they are listening to installers and customers the prices have remained pretty stable and availability is excellent.
P.s for optimum efficiency you should have the hot water set as priority.
The radiators should be oversized and run opentherm with low temperature heating, it will make quite the difference to the gas bills. Some say it doesn't some say it does, I've got got it and works a treat and my gas bills dropped dramatically when switching from old boiler pumping 70 degrees to the rads, and also don't need to worry about my little one scalding himself on the rads. Your underfloor heating will be low temp aswell anyway so really wouldn't be much of a change. I've got the evohome, the first 2 years were great, the. The 3rd year it started playing up and took me a little while to figure it out. The installer came out a few times aswell and even he struggled to work out what was wrong with it, ended up buying another unit which had the same problems just had very little control of it, and then all of a sudden it went back to working properly again. The evohome is a ball ache to setup, instructions aren't clear and now I want to buy some rad stats not only do I have to set the heating up from scratch once again (which will likely be problematic) but there also £75 per unit which is quite a lot when buying for 10 rads.
Anyway I've gone off kilter at the end a bit there.
