It is surprising what one can get away with. My son in a small house did last winter with two oil filled radiators only at 2 kW each.
My house much larger was designed to be heated by a single 4.5 kW gas fire. Open plan allowed heat to reach all rooms from one central fire.
This was latter used as emergency heating when a 20 kW boiler was installed with another 21 kW boiler for the hot water. But these boilers don't run 24/7 they only fire up when required.
Clearly power required depends on power lost so much depends on the insulation properties of the flat. Also of course ventilation tumble driers, cooker hoods, and bathroom extractors all pump your heat outside. Using heat recovery units can clearly help.
So in the main electric heaters will be thermostatic controlled and will not run 24/7 even in worse conditions. Using a 1 kW heater on a ring final is unlikely to cause a problem but a 3 kW heater could cause overloads.
The advice is any fixed appliance over 2 kW should be on a dedicated supply to avoid overloading the ring final yet the whole idea of the ring final designed during the second world war was to heat the home with electric fires. However in 1952 in the housing boom we did not have washing machines, tumble driers, and even electric cookers were rare.
So as to too small most rooms will only require 750 watt of heating assuming heat not lost to other rooms and modern insulated home without extractor fans running.
As to spurs off a socket this is more complex. We have a number of methods today to supply sockets. 1) The ring final normally supplied from a 32A overload which uses 2.5 mm sq cable the ring must be maintained to prevent overload and one is allowed one spur from each socket but that must supply a device which has a 13A fuse be that fuse in a FCU (fused connection unit) or a 13A plug. 2) The radial could be 2.5, 4, or 6 mm sq cable and may have a 16, 20 or 32 amp overload with over 2.5 mm sq simply not enough room to spur with many sockets if there is room then only one socket just like the ring. With 2.5 mm sq and a 16 or 20 amp overload you can add as many sockets as you like as long as volt drop and earth loop impedance limits are not breached. 3) The fused spur, well really it is a 13A radial and as with 16 and 20 amp radials you can add as many sockets as you like.
The idea with sockets is if overloaded it must blow a fuse or trip an overload. This does not mean you can add any load you want all it means is should you add too much it will fail safe.
Since no idea of system in your flat can't answer question.
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