1) First of all I think the import of slaves from Africa (or other foreign places) would have stopped by now as population reached saturation. I think there would be fewer ethnic minorities in the UK than in the US, because the UK population is already dense and there is already a large lower class to be tapped.
2) I think the majority of slaves now would be owned by corporations rather than individuals. They’d be forced to work in factories, call-centres, McDonalds outlets, and do other menial jobs like cleaning.
3) Slaves would not be used as soldiers. Putting guns in the hands of a desperate, oppressed population would be disastrous. They’d either use them to try to escape, or they’d use them to kill themselves.
4) People in desperate poverty would volunteer for slavery as an alternative to unemployment. People from poor countries may choose slavery in Britain (or the US) as a better alternative than freedom in their own country.
5) Because it would be no longer cheaper to out-source work to other countries, I think the manufacturing industries of the west would survive. There would still be ship-building on the Tyne for instance.
I’m sure I’ll think of more. Anyone else have ideas?