Saving Interest, Forex Trading, Split Year

Hi HMRC, 1. What should the saving interests from the bank overseas be regarded as in the (personal tax) self assessment? Income(SA100)? Overseas Income(SA106)? 2. What should the saving interest of GBP and USD (respectively) on an American (but UK-registered) online trading platform, Interactive Brokers (UK), be regarded as in the self assessment? Income(SA100)? Overseas Income (SA106)? or Capital Gains(SA108)? 3. What should the profit from spread betting in Forex Trading on Interactive Brokers (UK), be regarded as? Overseas Income (SA106)? or Capital Gains(SA108)? 4. What if we made profit from forex trading in the tax year 2023~2024, but suffer from a loss in the tax year 2024-2025 more than the gains in tax year 2023-2024? Does "Allowable Losses" apply in this case? Can both saving interest and gains from forex trading be regarded as Gains, when calculating the net gains/losses by deducting the allowable losses? 5. We had never been to the UK before. We entered the UK in mid-May 2023 and spend a month in Airbnb. We moved to our first residence in the UK in mid-Jun 2023. Is it the day when our tax year begins (split year treatment applies)?

Hi,
Interest arising in the UK, is declared on the SA100.
If you have untaxed foreign interest of less than £2000, you can also declare this on SA100, otherwise the foreign interest is declared on SA106.  
If you are trading as an individual and not a business, then you would declare a capital gain or a capital gain loss.  
Losses from listed shares and securities can either be carried forward ot set against other income in the same tax year. They cannot be carried back.
Thank you.