I don't know the specific legislation you are referring to, but as a rule ticket acceptance seems to be applied when there is a realistic chance of getting everyone to their destinations. In this instance the Avanti from Manchester will likely be packed to the rafters with valid ticket holders.
I've just checked and none of the trains from Manchester-Euston over the next few hours are showing as "sold out", which they would be were this true. Were I in the original poster's position and absolutely needing to travel, as that capacity exists, I would be tempted to try that PRO approach (ask for rerouting specifically mentioning the relevant legislation, be denied in writing, buy a ticket, bring a legal claim) though it isn't as I said completely without risk and it is a bit of a faff.
TOCs do not always arrange acceptance where this would be reasonable. While some have done in this case, "do not travel" is very often used as an excuse not to, particularly in the case of strikes but also in other cases, and neither of these matters are exceptions to the provisions of the PRO.
This link gives full details of the PRO - I've not quoted it in full because it's rather long but have quoted a summary section of its provisions:
Provisions in the regulation
Listed below are the provisions contained in the regulation:
- articles 1- 3 cover subject matter, scope and definitions
- article 4: transport contract
- article 5: bicycles
- article 6: exclusion of waiver and stipulation of limits
- article 7: information regarding discontinuance of services
- article 8: information for passengers
- article 9: availability of tickets, through tickets and reservations
- article 10: travel information and reservation systems
- article 11: liability for passengers and luggage
- article 12: insurance
- article 13: advance payments of compensation to those injured or the dependants of those killed in an accident
- article 14: assistance in seeking compensation from third parties when a railway undertaking contests its own liability for an accident
- article 15: compensation in the event of delays, missed connections and cancellations
- article 16: reimbursement and re-routing in the event of delays
- article 17: compensation of the ticket price (refunds in the case of delays)
- article 18: assistance to passengers in the event of delays (such as meals, possible alternative transport and hotel accommodation)
- articles 19 and 20(1): rights of passengers with reduced mobility
- article 20 (2): rights of passengers with reduced mobility
- article 21: accessibility of stations and rolling stock
- article 22: assistance for persons with reduced mobility at stations
- article 23: assistance for persons with reduced mobility on trains
- article 24: conditions under which assistance is provided
- article 25: compensation in respect of mobility equipment – removal of any limits to liability
- article 26: Personal security of passengers
- article 27: complaints handling
- article 28: service quality standards
- article 29: information to passengers about their rights
- article 30: enforcement
www.gov.uk
In this case the relevant section is this:
Assistance to passengers in the event of delays (such as meals, possible alternative travel and hotel accommodation)
As set out in article 18, in the event of a delay, passengers must be kept informed of the situation by the railway undertaking or by the station manager including current estimated departure and arrival times as soon as such information is available.
In the case of delays over 60 minutes, the passenger shall be offered free of charge:
- meals and refreshments in reasonable relation to the waiting time if these are available on the train or station or can be reasonably[footnote 1] supplied
- hotel or other accommodation and transport between the station and the accommodation in cases where a stay of one or more nights becomes necessary, when physically possible
- transport from the train to the station if the train is blocked on the track, to an alternative departure point or the final destination of the service, where physically possible
Where the railway service cannot be provided, the railway undertakings shall provide alternative transport as soon as possible.
Where there is disruption, the railway undertaking shall certify the same on the ticket when requested by the passenger.
When discharging these duties, the railway undertaking shall pay particular attention to the needs of people with a disability and others with reduced mobility.
"Possible" isn't defined so would take a common-law definition. If seats are on sale on the Avanti West Coast service, which they are, then it is clearly "possible" to re-route passengers onto it by purchasing those tickets up to the point that they are "sold out" regardless of whether Avanti wants to offer formal acceptance or not - the tickets are on sale for anyone to purchase, including if another operator wants to purchase all of them - they simply have to pay the public rate, just as easyJet would have to do if you requested your legal right to be re-routed onto Ryanair in the event of severe disruption (the rights for air travel are very similar). That you can't necessarily do it for everyone on the same train isn't listed as a reason you can't do it at all, that's like saying you can't put people up in a hotel at all if you have 11 people and 10 rooms, you can clearly put 10 up.
PRO appears to stand for Passenger Rights and Obligations, by the way.