The BBC News item Barometric pressure in London 'highest in 300 years' at least begins with
Overnight, Sunday into Monday, London's Heathrow Airport recorded a barometric pressure of 1,049.6 millibars (mbar).
It's very likely the highest pressure ever recorded in London, with records dating back to 1692.
It links to this Met Office tweet
So far Mumbles Head in Wales appears to have recorded the highest pressure during this current settled spell, with 1050.5 hPa earlier this evening. This remains a little short of the 1050.9 hPa recorded at Benbecula 16.1.1957, and the UK record of 1053.6 hPa, Aberdeen 31.1.1902
and then continues:
Met Office hourly observation reports recorded 1050.3 mbar at Liscombe in Devon, at 2100 GMT on Sunday evening. 1,050.2 mbar was recorded at Dunkeswell in Devon, and 1,050.5 mbar at Mumbles, in South Wales, shortly after.
None of these measurements breach the 1,053.6 mbar recorded at Aberdeen Observatory at 2200 GMT on 31 January 1902, which remains the national record, but the events of the past 24 hours certainly marked the first time for over 60 years that 1,050 mbar has been attained anywhere in the British Isles, said Mr Burt.
To discuss these records it is necessary to compare barometric pressure measurements to 0.1 mbar accuracy out of 1000 mbar, which is at the 100 parts-per-million level.
Question: How are barometric pressure measurements traceable over centuries to 100 parts per million accuracy? What was the procedure used to calibrate barometric pressure centuries ago that could be reproduced and compared to today's standards, or is there a very old standard absolute barometer somewhere that is believed to have remained calibrated?