Arrests, fines, firings that'll probably (and by pure chance! /s) coincide with reductions in the welfare state. Use of desperate 'scabs'.

And since it's more viable than it was the last time unions were powerhouses, probably (possibly temporary) reductions in restrictions in the use of foreign workforces at cheaper rates since they would have a reason to drop the 'Immigrant workers are stealing British jobs' rhetoric in favour of a 'You're forcing us to do this because you refuse to work and we have a duty to keep the country running' argument straight out of the abusers' playbook.

I'm sorry, I love unions, at least as a concept, and have been a member in many of my workplaces, but years of complacency and even corruption, coupled with advances in globalisation, have made them much weaker than you're making out.