I have start off by saying your "so maybe your implausible asseveration is simply that: an implausible asseveration" applying I am lying is rather offensive. As you correctly say in an earlier post, "It depends on micro climates, cultivation techniques and luck".
I live in the bottom of a valley, it is much colder than R.A.F Chivenor (which is nearer the coast), where the MET office records their data. As you walk up West Lane to the village there is a point where you can feel the temperature suddenly rise by 5 degrees. You can literally take two steps back or forward and be in the two different zones. I was only talking about a ground frost on the grass not an air frost, but it was still frozen water, in July! You can tell when there will be a frost in summer. It follows rain earlier in the day that by evening time has cleared up leaving very clear blue skies and a gentle but cold northerly breeze. Even in warm weather there are always heavy dews. Last year there were very bad late frosts and an early spring, my Camellias, Rhododendrons, Salix, Hydrangeas and many roses all had the growths killed. If you stood right at the top of the garden and looked out over Halsdon nature reserve there was a pronounced line where the cold air had settled and the frost had burnt all the new growth from the oak trees. I struggle to grow Buddleias, hydrangeas, mahonias that people up in the village easily grow. If any Australians read this, I am embarrassed by the pathetic specimen of Eucalyptus gunii (it is a particularly hardy selection that the Forestry Commission grow), even with all the extra care and attention I have given to it planting it at the top of a south-west facing side of the garden, in seven years it has only grown as high as 3M.
Also in 2015 at Chivenor was the warmest December night ever recorded in the U.K. when the temperature did not fall below 14.4C.
This year is exceptional there has been almost no rain for a month and temperatures as high as 25C. There was a late winter with snow at the end of March but since then hardly any frosts, the roses have not looked better with only a handful having blackspot. In a bad year almost all of them have it to some degree and many completely defoliated. I could not have chosen a worse place to try and grow roses. However Hybrid Musk roses grow very well.
My cottage, http://dolton.org.uk/subpages/past/page9bi.html