All TOCs that offer seat reservations allow you to name which seat you would like by number. In most cases this can be done through the telesales or booking office channels. If adjusting the online channel would cost a disproportionate sum, given that the other two channels are available, it would not be a reasonable adjustment to require that the train company also offer the feature online.
Telesales costs more than online (call charges) - as such a Court would likely have issues...
A station booking office? So a disabled person has to go to a station to book their ticket... right...
I am pretty sure that the Equality Commission would have a field day with your arguements....