App Spark, I don't agree with that, even though the rules do say 10A + 30% of remainder plus 5A for a socket, I think that's a bit mean.
Consider the cooking of a christmas dinner.
Turkey in oven one; roast potatoes and stuffing in oven 2, sprouts on ring 1, pudding on ring 2, gravy on ring 3, parsnips on ring 4. With a 2kw kettle plugged in to the socket (if there is one) and the canapes under the grill (if there is one).
Although each of the heaters will be cycling on and off by its thermostat, the load won't really be as low as the chart suggests. I'd be looking more for a circuit that is capable of maintaining the max load, give or take "a bit"
The fairer use of diversity is to calculate the probable load on the meter tails, as it is less likely that, at the same time as the Christmas dinner is being cooked, the electric shower and the convector heaters will all be going full blast.