You're probably right - if you can find someone in the midlands then that would be the quickest way to get going.

Just a couple of thoughts for you...

You mention CNC USB - if that's the software from PlanetCNC - do you believe that this is compatible with your 6040? It's unusual to support that, and Mach3, and the board doesn't appear to be a PlanetCNC board.

Getting spindle speed control is generally straight forward if the board (Breadout Board, "BoB") supports this - but the images you've provided to date are not very clear. If supported, you'd have to change two parameters on the VFD as well (allowing spindle forward/reverse control over external wiring, and allowing spindle speed control over external wiring) to support this - that's straight forward, but you have a bigger issue of getting Mach3 working first.

You mention the spindle speed control through Mach3, but in the next breath mention motors (plural) not powered. I expect you're talking of the 3 axis stepper motors? Have you no control from Mach3 at all?, and are the motors unpowered (you can rotate them by hand), or just not moving (they've be very difficult to turn by hand if powered, unmoving). It's unusual, however, for the steppers to be unpowered outside of an e-stop situation.

It could be useful to explain exactly where you are here - it sounds like you've possibly recently obtained this machine - we need to understand your starting point, and what you have working, and how much you understand. I'm forming a view but it's too easy to overlook or guess at missing information.