I was just scanning the objective Lamb Weather Type data for the most anticyclonic April in the data series that started in 1871, as you do, and found that the April of 1938 was the most anticyclonic on record by a very large margin with 92% of the month registering some kind of A type. It’s strange how climate statistics can very occasionally throw up a complete outlier like April 1938 was. The reanalysis charts for the month reveals that high pressure was never far away from IONA for the whole month, and for much of the second and third weeks was centred bang on top of it.
It’s not surprising to find that April 1938 was also the driest on record in the UK, on the other hand it was only joint 24th sunniest since 1929, the sunniest being the April of last year which wasn’t particularly anticyclonic at all, so forget any correlation between mean pressure and total sunshine.
For completeness I have included the Monthly Weather Report for the month which has quite a lot to say about what an exceptional month it was.