There would definitely be some merit in standardising the discounts for disabled people travelling with essential companions:
Someone who is visually impaired or uses a wheelchair can get a 50% discount for both themselves and a companion, without a railcard, on Anytime tickets only. They can't get this discount if it's just the disabled person travelling.
Season ticket holders can also take a free essential companion, but get no discount on their own season.
Neither discount can be booked online and tickets must be purchased at a ticket office (or presumably from a conductor). The staff have to assess whether the traveller is eligible.
However, with a Disabled railcard, a disabled passenger can get a discount of 33% for themselves and a companion on all ticket types, and can get the discount even if they travel alone.
These discounts are available to railcard holders however they purchase tickets.
Eligibility is checked in advance by the railcard provider against evidence submitted.
Confusing? Definitely!
Personally, I'd like to see a flat rate of 50% discount for all Disabled Railcard holders on all tickets including Seasons, with a free companion if required. However, as a railcard holder myself, I'm just a bit biased.