Len, I am intrigued that you suggest a rather fickle measure of warmth - especially for those of you living in exposed, maritime locations - though an article on the timing of such events would be interesting. When I used to measure temperatures on the Welsh coast, the approach to 20C was a cause for occasional rejoicing, an that was usually in summer! People seem to expect warmth in spring and hence hold onto ridiculous definitions of spring, including the spring equinox.
I gauge it by looking for St. David's Day on the calendar :) It seems to be perfect climatically, for us in the south, at least. Alternatively, how about the 10-day running mean of mean daily temperature exceeding 6C - the grass growth threshold, or maybe a little higher? Maybe 6C but not allowed before mid-February?
Molesey (well into spring here).