Sometimes the agencies quote ltd company rates. These are slightly higher than PAYE rates, but you are self-employed and you don’t get paid for holidays.
It is true that driving will generally pay rates significantly better than minimum wage. Long hours also mean an extra improvement on the overall wage. It is debatable if the risk to reward ratio is correct though.
Bear in mind that you will also be dealing with a load at your destination(s). Some of the pay rates take into account how hard the work is. Sometimes you see class 2 work paying the same as class 1, but the class 2 driver has unloaded himself at eight or nine drops, whereas the class 1 driver has opened his curtains two or three times or put the vehicle on a couple of bays.
You cannot necessarily say when you will be finished and early starts are quite common. When it actually comes to getting up at 2.30am, it is significantly harder than just saying it. If you look around on these forums, you will find stories of 15 hour shifts. This is like an office worker starting at 9am and finishing at midnight. It does happen!
Sometimes transport offices will treat you very badly, but then again, once you are out on the road there isn’t really anyone to bother you (besides the other motorists / cyclists). It can be a solitary existence, but you have the opportunity to listen to the radio, music, podcasts etc.
There are a reasonable amount of regulations to abide by and the driver can be personally fined as well as the company. If you do fall foul of the law, you can potentially have your licence suspended and obviously that can leave you in a pickle. Traffic Commissioners are pretty hard-nosed and pleadings of mouths to feed are of no concern to them.