It is absolutely correct to say that you must start with some assumptions when designing a central heating system and you choose what are the minimum results/capacity/capability in your chosen worst case scenario. But it is necessary to over-design for each room to some extent and then expect that possibly somewhere there will be excess output capacity rather than a deficit. Unless the rooms are all the same size and with the same thermal characteristics then the radiators will vary in size, which is why I can never understand why people such as inhibitor producers and others refer to the number of radiators in a house. In my house I suspect the largest radiator has of the order of 5-7 times the output capacity of the smallest which also compared to the rest is clearly the smallest.
My reaction to your description was the same as The Builder's, that something has changed but on the other hand it may simply be the effect of weather and your expectations being higher this time around. It is possible that your system managed to cope last year but that this year the demand has been higher (colder and/or greater losses) so that this time it is being stretched beyond its capacity.
Even in 2021 central heating is something of a mystery in the UK. Part of this is because housing in the UK has extremely poor thermal characteristics which are in some cases impossible to accurately evaluate (from windows, doors, etc. to absence of insulation, central cooling, etc.), part of it is lack of understanding of the basics (including by the "experts") and part due to the way the heating is used (primarily when it is deliberately rationed). The concept of having decorative heating components is a peculiarly British preoccupation, heating for heating's sake has still not really caught on and in general heating has a rather low priority with parts/whole of houses routinely receiving intermittent heating only and others none at all. Dampness/condensation is pretty widespread and regular occurrences of discomfort close to universal. Design criteria in the UK are routinely set at quite modest thresholds which so often end up disappointing.