You can get good drivers which are much cheaper (£36.20) than those, for instance:
Cheap CNC! Wantai 4 PCS Stepper Motor Driver DQ860MA 80V 7.8A 256micro CNC Router Mill Cut Engraving Grind Foam Embroidery-in Motor Driver from Industry & Business on Aliexpress.com

You may get customs fees, but even if it's the full 20% it's still nowhere near the price of buying in England. I've bought quite a few of them and can confirm that they are good drivers. Since they are 80V you could use the same ones for both size motors, so this makes the price difference just the difference between ballscrews and steppers, which going by your numbers is £103.

The Nema 34 motors especially would benefit from better drivers, particularly higher voltage ones. Some more options, not nesscarily the cheapest, just the first I found:
New Leadshine AM882 Drive 80VDC 0.1A - 8.2 | eBay
2MA2278 CNC Stepper Driver For Nema34,Nema42,Nema51 Stepper Motors 7.8A 110/220V | eBay

If I had to use Nema 34 motors then I would use the 220V drivers since the high voltage will significantly increase the torque you get from the Nema 34 motors in the higher speed region, but they do cost almost 3 times as much as the first drivers I linked to...