Make a list of your rads, find a sheet of outputs (shouldn't be hard to get the right one as they're so new), use the highest Delta T (ie hot rads, dissipating most heat), and add them up. Then add an allowance for pipe losses (under the floor) of say 10%. (I've measured some systems at 20%!)
If you work backwards from the recovery times of current (ordinary, not fast) cylinders, you'll find that they must be able to pass energy into the water at a much higher rate than the 3kW (10,200 Btu/hr) which used to be used as a rule of thumb. More like 12kW, or higher.
(Call it cynicism or experience, but I expect you'll find that your rads only add up to 40,000, and if you did the sums properly for house heat losses, with outside temperatures down to minus 2º , the rads are too small. You might need to increase the size of one or two.)
Chew it over - think how constant the demand for HW could be in the morning and the effect this would have on the heating. My guess would be that you'll be more or less warm enough but the boiler will be flat out in the mornings when the weather's really cold. You can help it a bit by putting the HW on before the CH, or using a programmable thermostat to stop the house getting too cold at night.
So that's a definite maybe, then.
Regarding the cylinder, nobody's checking. Some bright spark might complain at house sale if you've lost the document saying it complies with part L, but I doubt it.
It used to be Greater Manchester then i moved 30miles and now i'm back in sunny lancashire.
