Iv’e seen many of these,they always seem to be at least a month out.I wouldn’t dare plant squashes/pumpkins,runners etc before the last week in May,I’m N East lincs by the way..
Average last frost date is a very misleading term, as it is implying that five years in ten it is later than that.
If you are feeding yourself, you want a date that is after the last frost 19 years in twenty or better I would have thought. And for safety, probably closer to one in fifty years.
10-20th March in NW London is probably about right as an average, but frosts in April are quite common here.