CGT from when?

I've recently opened an account with a US trading platform. I would love to be able to open an S&S ISA, but as an expat living in the Middle East, I don't have that option and the US broker seems the best option for me. However, I have a few questions regarding what would happen if I were to return to the UK. As far as I'm aware, before returning to the UK, I could cash in my portfolio and avoid paying any CGT. But what if I didn't want to or my portfolio had dropped in value?

Supposing I invested £10k over, say the next two years, and then suddenly had to return to the UK for work, I believe my trading platform would automatically transfer my portfolio to the UK branch of its company, making me liable to CGT in the future.

Would the value of my portfolio be charged CGT only after any increase in value from the date I returned to the UK? For example, if my portfolio was worth £12k when I returned, then grew to £20k over the next five years, would I pay CGT on the £8k growth or the whole £10k growth since my original investment?

What if my portfolio was valued at less than my original investment, say £8k? Would I only start to pay CGT after it had gone back above £10k? Or would I have to pay CGT on any of crease from £8k?

Any advice much appreciated.