I'm going for 19C (rounded) to be hit somewhere in the UK today. GFS, MRF, Arome etc all forecasting maxes 4-5C above today's.
Given 18 places hit 13 today, 1 hit 14 (Aboyne, I'm assuming with some Foehn help), tomorrow's airmass is 4-5C warmer at 850hpa than today's and there look like being good breaks in the cloud in the middle of the day in the SE, I'll say 19C in one of Charlwood, Bournemouth, Exeter or St James park.
Brockley, South East London 30m asl