Meanwhile I need to google the term "dispensation" for I have no idea what it means
Dispensation, derogation and determination are different types of exemption from compliance with law (I haven't yet properly looked into the difference between them).
According to the Persons with Reduced Mobility - Technical Specifications for Interoperability non-compliance
dispensation letters from the Department for Transport, Northern are allowed to run their remaining 24 142s (they had 79 at the start of 2019) until 31/5/20 but only coupled to compliant (modified) 15x units, Transport for Wales are allowed to run their 15 142s and 15 143s until 31/7/20 without any restrictions, Northern are allowed to run their 23 144s until 31/8/20 but only on specified routes, and Great Western Railway are allowed to run their 8 143s until 31/12/20 but only coupled to compliant (modified) 15x units.
Derogation on the other hand, I believe is a more permanent thing, such as the DfT derogation letters for a couple of 170 subclasses that state that they need not comply with one or two minor things, but without a time limit / expiration date, so
ever.
Need to look into
determination.