> In the UK tax on interest earned on plain savings accounts isn't deducted at source

Yes, but as you point out you should really be using the ISA wrapper or a pension of some sort for most investments. I suspect most of those doing self assessment will either (a) be in the upper 25% of earners or (b) self-employed (~4.5m people).

It is annoying how the government has chosen to make tax slightly more annoying with "making tax digital" for self-employed people with quarterly reporting.