I’m not really sure why Bath is relevant given there were no speed restrictions imposed in the area. Perhaps that actually proves my point though. Bath has had 44mm of rainfall in the last 24 hours. That is 50% of the average rainfall historically in the last 31 days. Overall in the last 31 days we’ve had 176mm, over double the average. Despite this amount of rain there weren’t restrictions, but the line has closed due to flooding. In fact the Bath Claverton gauge you mention shows 53mm of rainfall now, as mentioned by others the severe weather and restrictions had not even started when you posted.
The forecast that was being worked off predicted up to 80mm of rain in some areas of the East Midlands, in high areas of South East Wales it was predicted there could be 150mm of rain. That is what I would call huge, especially in the time period it came down.
Rainfall gauges are helpful, but can only show the rainfall in one area, not the entire catchment area of the railway. However, as you mention them, within 4 I found the Shiptom Moyne Guage showing 108mm in the last 24 hours, 61mm of which in the last 6 hours. That certainly is a huge amount of rain.
As a note on the link you provided, I was looking at it this morning for work, only a handful of river levels were displaying dark blue (above normal level, flooding possible), if you look now you can clearly see there are a significant amount.
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