Apologies if you’ve already said, but have you passed a Class 2 assessment at a Royal Mail site yet?
If you have and want the Class 2 experience of reversing until you get more Class 2 shifts, when you finish a 7.5T shift early (Fairly uncommon to actually finish a shift on time or late unless you get held up in traffic from my experience - You’ll still get paid til the end of your booking time if you do finish early) You could ask to take one of the 18 tonners out around the yard to practice reversing onto a bay etc.
Generally I’ve found the majority of shift managers started out as drivers beforehand so might be more likely to let you have a go. Some of them have never been in a truck and it does become apparent when they ask you to do something that just isn’t possible.
It is you local Manpower office who allocates the shifts to agency staff. RM simply give them a list of shifts to be covered, so if you’re not given Class 2 - It is likely your local office doing that. You can remind them you are available over weekends and set you as “Available” so if the agency out of hours team need a driver, you’ll be on their list to pick from…
I’m down as a Class 1 driver on the agency but they still give me Class 2 and 7.5Ton work at RM, it just depends what work is available around the days/nights I want to work. As an example last week I did Tues-Fri on 7.5T and asked to work the Monday on Class 2. The difference between 7.5T and Class 2 is generally around £2.50 per hour ish looking at my previously paid rates but from my experience you aren’t likely to finish your shift much earlier than your booking time. So your £13 an hour on 7.5T is probably more likely to be £15.50 ish on Class 2. The parity pay is slightly more on Class 2 compared to 7.5T from memory but still low.