If you find your local Steel stockholder you'll buy 80 x 45 x 4mm wall in 7.5mtr lengths for about £60+vat. Depending on size 1 or at most 2 lengths will get you the base frame.
For the aluminum for the Z-axis and other areas of the machine, If you have any local engineering places nearby it's worth calling in and asking if they have an offcuts bin. Often what they call scrap pieces we call a Z-axis and they will sell you it at scrap value or if you drop on right bloke a few pints.
Drop on the right place and you might get steel off cuts their too.?

With £1200 it's just about doable to build a decent machine if you are careful and don't waste money buying the wrong stuff. Like has been said stay away from any electronics kit as they are nearly always made up of rubbish and poorly matched.

Linux CNC with a BOB will get you going cheaply like as been said and then it opens up a whole world of expansion if you move to Mesa, it will also open up a whole world of head-scratching as well depending on your skills and temperament learning to speak all thing Linux CNC but to kick you off cheaply it's perfect.

The secret to the successful building of a good machine is to NOT RUSH or BUY ANYTHING and always ask before buying because often someone will point you to a cheaper or better option.

If you take the Build route then the first thing to do is start a Build thread, even before you lay paper to pen, and ask ALL your questions in this thread. Don't start new threads asking questions because they get lost with time, keeping all your questions in one place makes it easier months later when your looking for reply. It also keeps your thread high on the trending list where you are more likely to get replies.

Good luck and don't waste money on those rubbish Kits as you will regret it.