Thanks for the advice, I'll start comparing ISA options.
With US investments is it still an issue if the fund I'm buying is in GBP?
You're mixing two concepts, currency and stock value. Convert the price to pounds to see if something is 'expensive' or not.
You can safely ignore all that, though. You need an asset alloc (%age of each asset type/location), then just buy to meet it. If US stocks are strong and the US$ is strong then yeah, in pounds, US stuff will be expensive. But US stocks could keep going up and the US$ could get stronger.
Roughly 50% of the world's market cap is US based stocks, but many of the large ones are of course international, so owning them is fine.
Your asset alloc is fine, actually. But yeah, ISA first.
Also re 'raiding your ISA for a house deposit' - DON'T. When you withdraw from an ISA that tax shelter room is LOST FOREVER.
Ideally start a house fund separately, in a few years closer to when you want to buy. Say 3-4 years out. In the mean time pump every penny into the ISA and leave it alone!