Thanks for all the replies. I got back from the Isle of Wight late last night after all passenger ferries were cancelled and I had to get to a different part of the island to get on the car ferry. Then quite a hike to Portsmouth Harbour station from where it docks. Rail replacement bus driver from Portsmouth Harbour (travelling only to Portsmouth & Southsea) was good enough to drive me on to Fratton where the last train to Brighton was starting from. I caught it with five minutes to spare. (SW Railway trains were leaving from PMS, so not sure why the Brighton train didn't).
Anyhow, back to the topic ... I'd rather not miss my booked 17:58 train from Preston to London Euston and have a potentially expensive and likely stressful wrangle to get on the next one (my ultimate destination will be Brighton).
Blackpool South would have been convenient because of where I'm staying. It seem though that there is no ticket office there (
and no ticket machines, incidentally).
On arrival into Blackpool North on my outward journey on Tue 1st March, I will try to get my ticket endorsed to be able to travel to Preston one hour earlier on the 16:13. Northern seems to have a horrendous reputation from what I've read here -- I don't know if that also applies to their ticket office staff. Or could I sort this out at Brighton ticket office beforehand? (I bought the ticket online with Chiltern Railways and received them through the post).