I have had some similar experience in a few different workplaces.
I think you are doing the right things by all leaving together, all taking lunch and letting the work pile up. In this case I don't even think that a deliberate "go slow" is necessary from the sounds of it, if just taking all the breaks that you are entitled to is enough to let the work pile up. If they give you shit for it, you could simply tell them that you are entitled to your breaks, and that resting will help you be able to work at full capacity when you are on the clock.
If you can collectively refuse to do unpaid overtime for long enough, the employer will be forced to either sign off on paid overtime in advance, or hire an extra hand. Forcing paid overtime in my experience can do alright at forcing employers to address understaffing issues.
Just make sure that you all take your breaks and leave on time consistently. If one person is doing it but not others, management/supervisors will likely see it as a personal rather than systematic problem, and try to put pressure on the one who leaves or takes breaks.



at work at the moment the team of 3 of us are having to work through lunch and work late just to kinda keep up with the workload being put on us. we don't get paid as overtime has to be signed off in advance. it's putting a lot of strain on us, one of the others has been going straight from work to her 2nd job. now obviously we have the 'right' to take lunch and leave on time, but that's nothing without the ability/power to excercise it.
so far we've managed to 'collectivise' the problem a bit, trying to leave together, all take lunch or none take lunch etc as previously the most experienced person tended to take responsibility on herself and work silly hours. we've discussed it and all agreed it can't go on, and are going to talk to the boss about it together and say we're overworked/understaffed and they should take on another person. they probably won't, and we've talked about all starting to take our lunches and leaving bang on time and just letting the work pile up. whether this will happen i don't know.
now it's a bit depressing it takes collective action even to work your contracted hours, but there you go. so does anyone have experience of similar situations? any tips on making demands of your boss?
fwiw the manager of another department joked we should just go on strike today as 'it worked for those tanker drivers'