This sort of combination tends not to happen much after about the
beginning of April, because, I imagine, the strength of the sun rises
temperatures to above average levels by default even if the air mass
is cold. The required conditions for such a combination seem to be a
reasonably strong wind from a cold direction (typically NE) plus high
pressure. This year we did have a late cold sunny day on the 4th in
Hampshire, with tempeature no higher than 12C, a typical March value
these days.

I also remember a similar windy anticyclonic NE period in mid May 1996
when the temperature was around 14C, 3C below 'old' normal values and
perhaps 4C beyond post-1989 mean May max temps. But has there been any
later? I was thinking next week might be a good setup but cold highs
tend to be cloudy through the months of June,July and August.

On a related point, when's the latest occasion of warm, sunny weather?
I remember some warm sunny days in early November in one or two years,
and some in mid January but never in December.

By sunny I mean settled conditions where the sunny conditions are
widespread BTW, rather than localised (e.g. coast or mountain)
effects.

Nick