Certainly under Virgin Trains, once it was known one of their trains was cancelled, they'd let you on the next train they were running going to where you needed to go. This would even be before (or a few times in my case, several trains before my booked one). I do not know if AWC are as accommodating.
Certainly, you are able to take the train before* or after your cancelled one. Trains to Manchester are very frequent, although today's timetable is subject to change. Also, if you arrive at you destination 15 or more minutes take due to a delay, missed connection or cancelled train, keep hold of your tickets and claim compensation. On a route like Euston to Manchester with a frequent service, a c20 minute delay getting the next train might not be too taxing on your time, but be a handsome payment in compensation.
*I have seen it here many times people saying you can take the train before or after your cancelled one. However, although I would love this to be true, I've been told otherwise on the round that it is the train after. If anyone has any documentation to help back up the claim its either side please let me know so I can carry this when I travel.
I have never had any problems getting refunds on 2 advance singles (one each way) when one leg has been cancelled and I've decided not to make the journey at all. On one occasion I the retailer said no because it was two single journeys. However, once I challenged this, saying it was two singles to make a return journey and one way would serve no purpose, they paid up with a fee free refund. Some retailers are more accommodating then others.