The installer or designer of the UF heating system must have calculated the heat output of his system . This total output number is the maximum demand you will have on the boiler for heating purposes . Check this kW/h number with the Sedbuk list and see what fits into this range , the choice is big . For the shower,taps and bath you are usually advised to take some extra capacity.
But this usually not necessary if going for a combi boiler since there would't be a demanding storage tank ( the hot press ) involved . A combi boiler gives hot domestic water AND heating water , although not at the same moment .Some larger commercial combi boilers would !
UFHs are slow reacting , so it doesn't matter at all if you run the taps, showers and baths one or two hours per day . The house would still get comfortably warm . If you do the showers as well then you could throw out the electric showers and supply them with pressurised hot water from the combi boiler.
Most combi boilers would only suport 1 shower and 1 tap at the same time or one bath tub and a tap , usually around eleven liters per minute at around 60 degrees depending on the boiler. Larger combi boilers are available but are usually only used in appartment blocks , hotels , B & Bs ect. .So if it is only a large family home then keep one or two electric showers and suply the others with the combi boiler.Unless you go for watersaving shower heads and taps, reducing the demand down to around 3 liters per minute per head . Lidl and Argos and most plumbing supliers sell these . Go for automatic mixing valves as well, this would allow you to set the outlet temperature at lets say 60 degrees but would avoid scalding .
Since your boiler is not placed in the house insulate the pipes well . The standard 1" insulation is to thin for the purpose, the losses would be to high . Go for the 4" insulation and pack this in standard 110 mm sewer pipes to be placed under ground . Insulation makes no sense when compacted by soil or gravel. Once the air is squeezed out by the weight of the soil on top of it it won't insulate, once it is soaking wet like a sponge it won't do so either. And when rats and mice build a cosy home along your warm pipe work by digging out some of the insulation material it won't work either. There are some 110mm flexible plastic ductings available for the purpose , these come handy when working at bends. And the ends - where they enter/leave a building -should be sealed , either with plastic end caps or with cement to keep out water and mice .
But the boiler should be placed inside the heated building , frost could damage it in the garage if not in the running modus . Usually there is an anti frost circulation build in in the modern boiler, but you must keep the electricity on so this device can do its job . For energy saving reasons the boiler should be kept in the house anyhow, shorter supply routes save time ( it takes a while to run water from the boiler to the tap ) and installation money (digging) and energy as well .
And to Builder : A modulting boiler creates as much heat as necessary , not more . That is much more economical than running a boiler on full out put ( as a non modulating boiler only can do ) and then switching it off untill the temperature drops and then starting again ect..... A modulating boiler tries to run as long as possible but on a temperature that is demanded . The more frequently a boiler starts the more energy it consumes, the starting process is the most wastive part of a boiler run . As with cars as well for example, until the engine is warm it consumes a higher amount of fuel . So instead of turning on and off lets say 5 or 6 times per hour it only starts say about 2-3 times per hour . The modulating boilers come in a big range, for the single home go for the smallest minimum output , down to around 4 kw the better ones modulate . That would be about one radiator. As soon as the modulating boiler senses a higher demand-for example a second or third radiator turned on in the house -it automaticly adjusts the output, the hight of the flame .
That is like sensefull car driving, instead of speeding untill the last possible moment and then pushing down the breaks one can save considerable amounts of fuel and breakpads and tire rubber when adjusting the gas and let the car roll out . Both methods would bring you through town in the same time, but the latter is cheaper and quieter.