Multiply the number of amperes you want/need to have available by the line voltage. Then divide by 1000. I think the UK uses 240 volts for all household branch circuits; all lights and appliances.
Or you can take the expected load in watts and divide by 1000.
Example: A building for which you expect to need 100 amps at 240 volts would have a load of 24000 watts aka 24000 VA or 240 KVA.
In a factory or other (usually commercial) building with a lot of motors i.e inductive loads KVA is computed a little differently but for residential use (mainly lights and electronics as resistive loads) you can start with just volts and amperes or start with watts.
The UK probably has a code book that includes how to calculate the expected load. The USA does have a code book, the National Electric Code, and load calculation includes such things as 3 watts for every square foot of living space.
