iMeasure kWh/HDD data back to 2007, and given iMeasure's use of a 15.5°C base temperature, I see that most weeks have had an HDD of 84 (so temperatures at/over 3.5°C) with the worst week (early January 2010) at an HDD 116 implying a mean 24h temperature around -1°C. A mean 24h (ie day and night) temperature of 2°C would imply an HDD ~95, and that is pretty far to the right of the chart; some years have no weeks that cold (eg 2014's maximum HDD week is 70).
Update 2015-11-22: down to ~0°C last night, which is almost the worst case for winter, and we see a temperature drop of ~3°C overnight in the occupied (1g) bedroom, which while not "no heating required" is acceptable. Before lights out temperature was over ~18°C on all walls at eye height, internal and external walls, though down to ~14°C on the external wall behind furniture, so that may need an eye kept on it, see graph.
Update 2015-11-30: works started in the master bedroom and off to a good start with the old plasterboard off well before lunch and the aerogel going by end of day. Lots of foam used to make air-tight. There were visible gaps round the new 3G windows again.
Update 2015-12-02: again it seems that our HDD base temperature really is close to 12°C, eg in this graph, see that the temperature of occupied rooms did not fall overnight, only the currently-unoccupied study (5s).
Update 2015-12-08: plaster only ~50% fully dry and temperature dropping to ~5°C overnight, so closed the windows and put small dehumidifier in room to help get ready for painting.
Update 2015-12-10: plastering was on 3rd and 4th, and the the green crosses on the night of the 4th onwards show the relative humidity in the room, from the evening of the 6th with the sensor sitting in the wall in a vacant socket. The plaster nearly dry as the humidity dropping below 80% with the dehumidifier running for the the second 24h (having filled and stopped overnight/morning of the 9th). This graph shows a differential from outside of ~9°C being maintained by the dehumidifier's nominal 183W power, so ~0.05K/W ie 20W to maintain 1°C over external temperature; the 183W is about the heat output of two people, though note that this dehumidifier is a heat-pump with a CoP a little over 1, and also that there are no curtains present yet which may understate final K/W.
Update 2015-12-16: ... and we're done! (We actually got the bed back in last night but there were some finishing touches needed such as rehanging the door, the window reveal, and some paint touch-ups. Furniture and other stuff will go back in gradually, but already the decluttering of the rest of the house is noticeable.
2015-12-18: Building control visited to sign off the work.
All including VAT at full rate.
A useful comparison would be a construction infilling the studs (to the full 100mm if necessary) with some sort of non-vapour-check foam, and then having 40mm of foil-backed foam (eg Kingspan Kooltherm) over the studs where the aerogel is now. Questions for that would be: