Contemporary Twitter feeds (20th/21st October) have appeared on railforums, which add a little more background. I can't say how complete the record is, only record what I see.
The bad weather began on 19th, with blanket speed restrictions, but I don't recall any actual flooding. On 20th, this message continued through the day, and it does seem that the cancellations (some partial) were due to slow running. There were warnings of disruption, with these messages just after 16:30:
Due to severe weather trains have to run at reduced speed on all lines. Train services running across the whole South Western Railway network may be cancelled, delayed or revised."
"Disruption is expected until the end of the day. Your journey is likely to take longer and you may need to use a replacement bus to complete your journey.
A band of strong winds and heavy rain are currently moving across our network. Speed restrictions are currently in place on these routes:
- Exeter to Salisbury
- Weymouth to Dorchester
Then after 21:30 (too late to help in this case):
We've received reports of heavy rain flooding the railway between Yeovil Junction and Exeter St David's Train services may be disrupted through this area.
On the 21st, the bad news started from the beginning, but the speed restrictions were expected to end soon - 12:00, a time that slipped later throughout the day. Before 10:00 the message was starker:
We recommend that you do not travel on our services between Exeter St David’s and Salisbury until 14:00. At this time, we estimate that disruption to our services will end at 16:00
Due to flooding between Exeter St David’s and Salisbury ❗ Do not travel on our services between Exeter & Salisbury
Information source Your ticket will be valid to travel on services tomorrow (21/10/21)
Ticket acceptance on Stagecoach 4, 4a, 9 & 9a Exeter - Axminster via Honiton
There were promises of ticket acceptance by
GWR▸ , naming a variety of routes, and then a general "any reasonable route" one at 17:00. At 17:37:
We're currently experiencing issues with sourcing replacement transport on this route. If you are able to source your own taxi, you can claim any expenses back from our Customer Service team online:
https://bit.ly/3aZ2N7K or by post: Freepost
SWR» CUSTOMER RELATIONS
and 6:05
We strongly advise that you do not travel to stations between Yeovil Junction and Exeter St David's as you may not complete your journey.
Assuming the relevant day is 20th, the relevance of what was put out on 21st is the absence of that the day before (assuming that's a valid inference from the recovered feed). It looks as if SWR expected to cope, until the flooding late in the day meant they could not. It is also worth noting that the messages being put out on Twitter should have been matched by what was on show at Waterloo. But if this lad joined the train at CLJ, how much of that was even visible, let alone prominent?