Scott's pneumatic setup is:

Tanks- Custom made
Valve- VM1000 (from RS components)
Ram- Lego 48mm (from ebay or bricklink)

I'd say you'll struggle to get something working reliably within your budget. I've been working on a pneumatic ant and never got it to work reliably and effectively. It's pretty tough, Antweights have been going for about 13 years, and there have only ever been 2 decent pneumatic ants. Lots of people have tried, but they always fail or aren't that spectacular. Mind you, having said that, the most successful antweight ever is a pneumatic flipper, so when they work, they work very well...

Also, why avoid the "ebay specials"? With a budget of £70 you won't do better than a pair of them for a tenner. Smaller motors will be more expensive and less powerful.

I'd recommend getting the basics first, like transmitter, receiver, battery, battery charger, speed controller, motors & wheels. Even if you get all the cheapest stuff available, that's still going to eat up most (if not all) of your budget. Then once you have a moving chassis, then start work on the pneumatic system. It's no good having a kick ass pneumatic system if you don't have a robot to fit them to.

But if you do have your heart set on pneumatics on a budget, get on bricklink.com and see what's for sale. You can probably get enough for a basic lego pneumatic system (tank, ram, valve, pump & tubes) for about £20, or even less if you're able to make your own tanks. But a word of warning, that is more likely to go boom (It was a failure on the custom tanks of Scott's antweight in that link that you posted).