Skip hire firms end up with their skips full of customers' 'rubbish', much of which (including metals) can be recycled by the hire company. So skip hire companies will always have a close relationship with scrap metal dealers and vehicle recycling firms.
If they're coming to you, and paying in cash (as most firms would probably do), you've probably got little to lose. For example, if you phoned them up and they promised you £220 but only offered £50 when they arrived with their low-loader, you could simply tell them to get lost without losing anything.
Chris