I've been reading up on this. From what I understand so far (I'm not a lawyer so please correct me if I'm wrong)

Capital gains: paid when you sell an overseas asset. If you are not a tax resident when you sell the asset (I.e you have moved back), you are not liable to tax. Any capital gains made while you are a resident are.

When it comes to etfs and such, make sure you only invest in "reporting funds". If you make capital gains they are taxed as capital gains (versus income, which non reporting funds are considered and taxed at a higher rate.). There is a list of these online provided by the HMRC.

Dividends: taxed as income, the same way dividends are taxed here. If you have under £2,000 you don't have to declare or pay.

filing: you declare these by filing a self assessment tax return. You may not have to pay tax if there is a dual taxation agreement, but the UK also respects this unilaterally if one doesn't exist.

I'm not a lawyer or expert, so I'm going to get a tax attorney.