I see the full contents of Met Office Monthly Weather Reports are now available on their digital archive here:
https://digital.nmla.metoffice.gov.uk/a ... 210a926e1/
under 'Monthly Weather Report Not Quality Controlled' (previously it was just the General front page Summaries).
I think the proviso refers to the fact that there will be a few historic extremes which have since been re-assessed and revised/erased (some noted in the Corrections/Additions pages in later issues)
One caution - the files are quite large!
A really useful resource and credit to the Met Office Archives for adding this