There is legislation against late payment - it doesn't get enforced very often, but it's very much in favour of the invoicer.
The only problem then being that you get that one invoice paid reasonably quickly and then get delisted and never work for them again. It's nothing short of bullying.
I worked in central accounts for an enormous international company - the biggest in their sector - and they had a scheme whereby every outlet they oversaw (thousands) not only had to do the job running a busy outlet, but also had an on-site console-type thing they had to use to process their own invoices, before anybody centrally even knew of the existence of the invoice. Of course, this was difficult enough for them to find time to do along with their actual job, but they also had to get suppliers set up, which took a very long time, before they would even appear on their system for them to be able to process their invoice. The payment terms were considered to begin only once the outlet had been able to process their invoice, even if they'd been given it weeks previously. Sole-trader suppliers were given our number to call for payment, but then told to go back to 'their customer' (the outlet, which was of course also us) to chase them.
There were processing cut-off dates for the end of the month (sometimes as early as the 16th), meaning that the invoice was effectively counted as a next-month invoice.
Some of them were tearing their hair out when they expected (had often been told by a desperate/lying/optimistic outlet manager) payment in the next week or so and were told that, once their invoice had been entered on-site, it would miss the cut-off and so would be paid 90 days after the end of the following month.
Any pleading from frightened, desperate people - many of whom only worked for us and should have been given the protection of employment status, but were forced to invoice on a self-employed basis, was batted back to the outlet or they were told they weren't on the approved supplier list, so basically should be grateful if they get paid at all, as the outlet 'shouldn't have used them'.
Naturally, the approved supplier list was heavily weighted in favour of the big boys and they were kept sweet, just as in turn they kept us sweet with preferential cut-backs. I got regular calls from a taxi driver on a very remote Hebridean island. Every time he tried to fight to be paid what were basically his wages, he was told that the company had an approved national taxi company and he was not it. Astonishingly, this 'nationwide' taxi company did not have any presence on this tiny little remote island (which was home to a small but quite important government outpost). There was one taxi driver on the island and it was him.
We were given some leeway to bring payment dates forward (supposedly only to correct late processing at the outlets) or if people really kicked off, but I just did it anyway. I gathered a spreadsheet of small one-man-band operations and those who should really have been on the company payroll and ran them through various check screens to get them included in the next possible payment. I figured that, when monthly invoice payments would be in the tens of millions, £20-30K of effectively wages wouldn't even be noticed. Nobody ever told me off about it.
It was soul-destroying when (justifiably) angry and desperate people who thought I had far more power than I did would call and ask if I thought it was acceptable that they had to wait so long and my honest feelings were that no, it most definitely was not fair.
The absolute nadir of it all was that I was in Accounts Payable receiving calls from sole-traders, having to tell them that their strict 14-day terms were being ignored and we would pay them in 90 days (and then only if nobody delayed at any stage) and tough crap if they didn't like it. Further up the office was Credit Control and I didn't realise for quite a long time until I spoke to one of the department who mentioned it, that they were calling the same 'status' of people - sometimes the exact same people who'd just spoken to me chasing their invoice - because we'd supplied them with a meal or some other sundry item that we'd invoiced - and tell them (not an idle threat) that they had to pay in 7 days or else they'd incur late payment charges and feel the full force of our legal department. Any (very reasonable) protests of "But can't you take the £10 I've owed you for a week off the £900 you've owed me for five months?" were met with "It doesn't work like that".
It's arrogance and bullying of the most disgusting kind. I left soon after that.