Hi James T,
A detailed overview of the capital allowances that can be claimed by self employed individuals is provided at:
Claim capital allowances
Capital allowances can be claimed by completing the relevant boxes in the Self Employed pages (SA103 or SA103s) of the Self Assessment Tax return.
Self employment (short) notes (2022)
Income received by self employed UK residents from overseas clients should also be reported in the Self Employed pages.
Any Foreign Tax Credit Relief due re: foreign tax already paid can be claimed in the relevant section ('Foreign tax paid on employment, self-employment and other income') of the Foreign pages (SA106).
Self Assessment: Foreign (SA106)