Hi
I know many on this forum enjoy crunching numbers.
I have several years worth of gas usage data and weather data from a thermometer in our garden so I decided to correlate one against the other using Excel (for entertainment really - yes really!).

The result? An indication of our house's energy requirement.

If you want to give it a go with your data (again for entertainment!) I have made it available at Heating estimate using nearby degree day data.xlxs

One observation is the large scatter on the data.

We do get a lot of thermal gain on sunny days. Our heating thermostat, in the hall, on a north facing outside wall is not far from an air-brick and the house does feels warmer on days when there is a brisk Northerly wind! It is also apparent that we use more hot water in the winter (colder incoming supply, showers in the summer, baths in the winter). Also cooking lunches tends to leak heat into hall often resulting in a "cool" period for an hour or so after lunch.

I was impressed by hydrosam's comprehensive article Correctly sizing Air Source Heat Pumps (ASHP) - Bigger is not always better and https://energy-stats.uk/what-size-heat-pump/ was also highly informative. I'll have to try out some of assessment methods by way of comparison.

So, is this a way to obtain the Heat Transfer Coefficient (HTC)?