Weight weenie bike on a budget, this should be interesting

A lot of people think cheap and lightweight don't really go together, i think i'll be able to go under 7kg without spending over £300-350

Start big then, alu frame like a second hand CAAD10 are around 1050g.

Once you get the frame sorted, the rest should be a bit easier to get around 7kg.

After you've build it up, you end up realising there's very much little difference in how the bike ride regardless of whether it's 9kg or 7kg.