Hi Je77 Jones,
We are unable to comment on scenarios.
You would need to review the guidance on statutory residence and take the tests here:
RDR3 here:
RDR3 Statutory Residence Test
If you are treated as tax resident for the full tax year, you will need to check if split year treatment applies.
If it does, then there would be no UK capital gains implications on currency transactions before you arrive in the UK.
Please also have a look at the guidance on foreign currency here:
CG78300 - Foreign currency: introduction
For a more indepth answer to a detailed question of this nature, you would need to contact our Self Assesment helpline or seek professional advice.
Self Assessment: general enquiries
Thank you.