Judging from an earlier post from Geordie_Flyer, there should be some sort of check-in link. Now it wasn't totally clear but it seems that was in MMB, and/or there may have been something in the email.

From what I can make out, it is built on an existing relationship that BA has with First Great Western. It has long been possible to book services from (e.g.) Exeter to Shanghai with a train sector on BA. So FGW appears to be used as a mechanism already in place, only drawing on National Rail sectors. Online check in for the flights is the normal process, but for the train (and coach) components OLCI starts at 72 hours. Incidentally train sectors do not normally award Avios or TPs so this will be an ORC, and we are now told that you do the ORC by claiming using the usual link in BA.com for missing credits on BAEC.

Does this link help? Use your surname and BA PNR.

https://check-in.accesrail.com/#/step1

Maybe one day BA can expand this a bit more for wider train + plane use UK wide.