I think that has already happened, unless my recollection is wrong.
I'm fairly sure that 20C or 21C was reached during the record-warm spell of mid-February 2019, around the same time that the previous record was set in 1998.
Yes, 2019 was exceptional with several record-breaking days. I remember sitting in a pub garden in a T-shirt on Sunday 24th and that wasn't even the warmest day.
From Trevor Harley's site:
On Thursday 21 Scotland's record maximum (since 1897) was broken, with 18.3C recorded at Aboyne (Aberdeenshire). On the 22nd a new Welsh record is set with 18.1C at Gogerddan (Cardigan Bay), and then broken again with 19.1 on the 24th also at Gogerddan. On Monday the 25th 20.6C broke the 20C record in Trawgoedd (and 20.4 in Northolt, London). Tuesday 26th was then exceptional: 21.2C at Kew Gardens in London, and 20.8 in Porthmadog (Wales). And of course the month started with deep snow on the ground! We had a a couple of inches overnight 31 Jan - 1 Feb and then a serious dump on the night of 1-2 Feb. It took me about an hour and a half to get the 4 miles home from Fleet station with my bike as the snow was too deep even to push the bike through easily. 2 Feb was a glorious day with 8 inches of snow over Basingstoke way and brilliant sunshine.
Feb 2019 was probably the best winter month of the last decade - it had everything.
Yateley, NE Hampshire, 73m asl
"But who wants to be foretold the weather? It is bad enough when it comes, without our having the misery of knowing about it beforehand." — Jerome K. Jerome