From 20 September there will be automatic acceptance of rail tickets by other companies during disruption- not waiting for it to be agreed or advertised, with any reasonable route able to be used.
This applies to all publicly owned (effectively re-nationalised) companies, and to all ticket types-
(The new means this had been quietly happening for some time, but has been expanded)
c2c
Gatwick Express
Greater Anglia
Great Northern
LNER
London Northwestern Railway
Northern
Southeastern
Southern
Thameslink
South Western Railway
Transpennine Express
West Midlands Railway
Avanti West Coast (new)
Chiltern Railways (new)
CrossCountry (new)
Great Western Railway (new)
This is the official government announcement- https://www.gov.uk/government/news/your-cancellations-covered-new-scheme-to-help-stranded-rail-passengers-arrives-this-weekend
As more companies are taken under public control the list will expand.
There is a lot of "devil in the detail" but for now this is the high level status.
As one example Kings Cross to York- if there are issues you can automatically go to Euston and catch an Avanti train to Manchester, then a Trans Pennine or Northern train to York (but NOT an East Midlands train from St Pancras to Sheffield yet).
Or from Gatwick if there are issues on, say, Thameslink and/or Southern you can use Gatwick Express.
Or, like this morning, the Paddington route to Reading was closed entirely due to an incident. so you can now automatically use Chiltern between Marylebone and Oxford, or South Western between Waterloo and Reading (or even Waterloo to Exeter).