Tannoy VX12

[walpurgis]

I am envisaging a 3 way with either a front mounted 12" or 15" side mounted woofer...

I'm not sure what you mean. The Tannoy is a full range driver, although you could use a subwoofer with it.

I would have zero knowledge of how to design a box and even less so how to sort out some kind of crossover for a woofer, assuming I use the existing one which looks like it was built by the military for the vx12 driver

Picking a suitable box design should not be too tricky. Bass can be extended below the driver resonance with a bit of port tuning. Port dimensions can be worked out once you pick a cabinet volume.

I can't see why you would not be able to use the existing crossover. You might want to use an alternative terminal plate equipped with banana sockets. That should be easy to sort.