Historic precipitation data is available through the Met Office Hadley Centre observations datasets: HadUKP.
HadUKP is a dataset series of UK regional precipitation, which incorporates the long-running England & Wales Precipitation (EWP) series beginning in 1766, the longest instrumental series of this kind in the world.
The data is available on a regional basis, so we have taken a copy of the South West daily totals data. These provide a daily precipitation value since 1931. This data is provided as a text file with a row for each month, values separated by spaces. A python script in this repository converts the data into a friendlier CSV file.
The date measured
Daily Southwest England & Wales precipitation (mm)
Data is made available under the Open Government Licence, with a requirement to attribute the original source, which is the Met Office Hadley Centre.