Well maybe my post was a little bit unclear.
In 1880, everything is still fine as carriages from ~1975 are still available for a few years.
However, after ~1880, most of the newly introduced carriages don't serve second/first class or they don't serve mail, so in ~1890, it will be very hard to create a train with mail and second/first class that is still available for at least the next 10 years.
In 1890 you have the following options:
You can take the Metropolitan eight wheel coaches, which don't have mail carriages.
GNR six wheel cars only have third class.
Four wheel coaches available until 1911 don't have mail, mail is only available until 1894 for these.
LBSCR six wheeler have first class only available until 1892, other classes will be available until 1901.
New LBSCR (1889) cars only have third class.
GWR corridor cars only have third class.
Others won't be available after 1897 at all.
A few years later, in 1895, things will be a bit better:
LNWR corridor cars have all cars that one could need but these are only available for 4 years (is this a typo?)
New L&TSR 6 wheel are available, which are available for 11 years, which is fine, however in 1895 only 7 years are left, which is not much but enough in case one needs more trains than expected on that line.
I don't know what exactly should be added as I don't know pretty much about british railway history.
However, without that in mind, I guess any of the following would solve the problem:
Create some Metropolitan or Metropolitan compatible mail car.
Create some GNR second/first class, may this be a combined car or two different cars.
Increase availability of some four wheel or compatible mail coach to 1911.
Increase availability of LBSCR six wheeler first class car to 1901.
Create some GWR corridor car compatible second/first class car, may this be a combined car or two different cars.
Increase LNWR corridor cars availability to be available for at least 10 years.