Nitroduck and PMI regs are good (WDP regs are rebadged PMI regs), and many people have said the Centerflag regs are better, although I've never used one. Last time I looked, Gov'nair regs used weird bottle threads which rules them out IMO.
As for bottle, I'd guess for under £100 including reg you're stuck with a 3000psi alloy bottle, which will be heavier than a fibre-wrapped one. For 3000psi, assume around 500 shots from a 0.8l bottle, 700 shots from a 1.1l (the amount will vary depending on marker efficiency.) I don't know which to recommend, but I'd have though a 1.1l would be manageable for most people. Then again, if you're playing recball and have the opportunity to refill fairly often, a 0.8l will be a little smaller and lighter.
If you start with a 4500psi reg on a 3000 psi bottle, you have the option to upgrade to a 4500psi fibre-wrapped bottle later (lighter, plus more shots per fill!)