I’m not so sure. Most people aren’t even aware that the 100-123K band exists, so the headline would be that the 45% threshold had been reduced from 150K to 100K. And like I say, it would probably affect people’s behaviour and result in a greater tax income to the treasury.
I’m suggestion not removing the allowance at all. Just make it a straight 45% at 100K, and they’d probably collectively more tax. As it is now, anyone on that threshold will have limited motivation to do any overtime and/or put any extra in legal salary sacrifices such as pension.
There is a threshold at £123,700; you currently pay 60% tax between 100-123.7K and 40 percent beyond that up to 150K. at which point you pay 45%.
As for self-employed status, I can't imagine Labour being any more friendly than the conservatives about this if you only work for one client.
If Labour increase PAYE taxes, then they'll almost certainly increase the corresponding dividend tax as well. Likewise, any reduction in tax band thresholds will apply to all declared income, not just salary.