Which bit can you not book ? I booked a sail & rail ticket to North Wales a few weeks ago. The ticket (or the receipt) was pretty clearly marked as being to 'zone a' or whichever zone it was.
Is it that they say you can't book to Glasgow in particular, or you can't book on the date you want to travel ?
I just looked at the link and I see it is for Larne-Stranraer sailings and so is different than the ticket I had booked, but the principle is the same.