That is exactly the system we have, oil condensing boiler, underfloor in the living room and kitchen, and rads in the hall, utility, and bedrooms. It works extremely well (for us). We don't have the underfloor on a 24/7 basis. I replaced the originally installed thermostats with programmable (heatmiser) thermostats, as also referenced in the article. This makes a big difference, and as we are both out during the day, I can step back the underfloor during the day and at night.
The underfloor takes around 1 hour to change 1 degree, so it never drops too low when not on. In fact, as the room is facing due south with a lot of glazing, we have in effect a massive heat sink in the floor so the underfloor doesn't even turn on for half the year.
As it is an oil boiler, it isn't variable, so it's 70 degrees or nothing (gas boilers can modulate). The "blending" is in fact a valve on the underfloor manifold, which mixes the 70 degree water with the returning underfloor water and pumps it back (using the pump on the manifold) to the underfloor pipes at around 45 degrees. As the underfloor isn't on all day, the water in the underfloor pipes is initially around 20 degrees, so the water returning to the boiler has enough delta for the boiler to be in condensing mode. At least until the temperature rises. But then the same can be said about radiators in rooms. Once the room is warm enough, the rad isn't losing enough heat to allow condensing mode.
Is the the most efficient? couldn't tell you, but it is the best and most realistic solution for us, given what we had to work with. And has proven pretty cheap to run. And the underfloor heat is joy instead of rads. Rads would not have worked in our living room, which is about 65 sq meters with two walls of glass and not a reasonable chance to hang rads on the remaining 1 & 1/2 walls.
The way I see it is that I have lovely radiant heat from the underfloor all the time I'm there in the living areas, and I have immediate heat from the rads for the half hour in the morning and half hour in the evening when I am in those rooms.
The manifold is in the boiler house with the boiler and water tanks (hot and cold).
The house is around 2000 sq ft bungalow, and we use around 2400ltr tanks of oil a year. It is pretty well insulated with 800mm of insulation in the attic and standard block wall construction with pumped insulation between the leaves. House is around 45 years old, but with upgrades. Two occupants, both out during the day.
I'm not a plumber/boiler technician/builder and don't play one on TV, but did a lot of research before we upgraded.