I very much doubt a driver is going to fill his lorry with illegals and transport them to the UK for nothing - but there you are. That's just my thoughts.
Some nasty comments on here. They were from Bulgaria. There is a thriving trade in workers from Bulgaria coming over, thinking they are going to get work, and they end up being Slaves. I doubt very much they had paid for their journey, more likely they were promised work. Its dreadful how they died and anyone insinuating they deserved it should be ashamed.
Maybe the driver went to the loo, and they all decided to pack themselves into his lorry, he could have been none the wiser. Alternatively, someone high up may have been paid, and he may be the poor chap who just drives the lorry.
Bulgarian nationals, as far as I'm aware, may come to work in the UK. They don't need to sneak in. The lorry came from Bulgaria but it doesn't follow that its cargo was loaded in Bulgaria.
OK - but I think these days most lorries coming from Europe are pretty secure - for obvious reasons. As I understand it drivers are liable to be fined if found to be carrying stowaways.