That kit is very poor mainly, as you said, because it includes long unsupported rails which are not suitable for a machine to cut aluminium. It also doesn't use ballscrews.

The place almost everyone here uses is linearmotionbearings2008 on eBay. His prices are much lower than anywhere in England and he will cut the rails and ballscrews to whatever lengths you require.

linearmotionbearings2008 | eBay

Which motors and drivers did you get exactly?

If you search 'aluminium extrusion supplier' on this forum you'll find some, but I would seriously consider using steel box section as it should be far cheaper or stronger for the same money.

I live near Leicester so I'm probably quite close...happy to meet and discuss things if you want to.