I do understand your question now - "Is there a tax free threshold for declaring foreign interest?"
35 years ago there was obvious official guidance for a tax free amount of foreign interest (£100 or so?) but I haven't seen that guidance for a number of years although I haven't combed the current guidance minutely. Perhaps someone can point it out if it still exists.
Establish if you are resident for tax by reading the Statutory Residency Test.
https://www.gov.uk/government/publications/rdr3-statutory-residence-test-srt/guidance-note-for-statutory-residence-test-srt-rdr3If, under SRT, you are non-resident, you may have options as to what you have to declare to HMRC. The number of days present in the UK is the determiner and can vary from 16, 30, 31, 46, or 183 days. There are 'split year' allowances.
Don't confuse the mention of non-residence via the SRT, on the GOV.UK sites, with non-dom status (remittance basis) for non-UKC Americans. They're two different things. If you have UK earned income, being taxed on a non-dom basis will restrict your UK Personal Allowance which means you will pay more tax on that UK income. Avoiding the "Arising Basis" (taxed on worldwide income) is usually not worth it unless the foreign income is somewhat substantial in relation to your UK income.
HMRC has tightened the rules on tax free income such as the 10% tax free amount for foreign pensions and, in spite of the PR from the government, is requiring more and more taxpayers to file the Self Assessment form. The self assessment form is actually very easy for most to complete and in no way is anything nearly as complicated as a US 1040 form. It usually takes me much less than an hour to complete mine including calculations, and I do mine the old fashioned way of pen and paper. All you do is fill in the relevant boxes where they apply. In your case (£30 foreign interest) you probably don't need the 'Foreign Pages', so even simpler. There is commercial software available as well as the HMRC filing Self Assessment site.