Looks like 0% for all items under 8525 under the new Global Tariff (i.e for countries with which the UK trades under WTO terms). It would be 0% for countries with an FTA as well, of course:

https://www.trade-tariff.service.gov.uk/headings/8525

The distinction in law between sub-30 and 30+ minutes still exists, inherited from EU law, but the tariff is 0% in both cases:

https://www.gov.uk/guidance/classifying-electrical-equipment...

Summary of the changes from the "Common External Tariff" of the EU to the "UK Global Tariff":

"The net effect of the tariff regime according to a summary by the Department for International Trade is as follows:

- 47% of products will be tariff-free, compared to 27% under the CET and

- Average tariffs will be reduced from 7.2% under the CET to 5.7% under the UKGT

This is achieved through changes to rates of tariff by lowering rates, reducing rates to zero or rounding rates down."