The issue is whatever is creating the actual motion (i.e. the actual pulses), be that the computer via it's PP or an external controller, has to be able to see the spindle pulses real time inorder to sync movement to the spindle pulse(s). Whenever you introduce things like USB or Ethernet into the mix, you introduce indeterminate delays (i.e. the delay varies depending on various factors which lead to random time delays), so you can't have for example the spindle pulses being inputed to the PP and expect an external motion controller to be able to sync to them as the delay would cause unreliable syncing.
For external motion controllers there are a few options. SS/ESS are probably the most popular at the cheaper end of the market, but you've got buy from the US or Aus (PMDX do them, and if you're needing a new BOB, PMDX are some of the best).
In the UK, Zapp Automation sells the russian controller which I've completely forgotten the name of (control logic???), and CSLabs stuff. Best to drop them an email to see what they've got in stock as website stock levels aren't always accurate.
Other UK option is Eaziform/ConquerorDesign, but none of their stuff is Mach compatible.
Everything else I can think of are all US based.