HMRC/HMT should stop collecting "Tax on Account" in January. It's already enough of a burden to self-assessment tax payers to pay the tax they owe, that they shouldn't additionally have to pay a speculative amount for future tax not yet due (which may be on earning not yet made).
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The concept of "Tax on Account" is similar to a savings plan. That is, to pre-pay some amount of money against a future bill.
However, for self-assessment tax payers (ie. all higher rate earners, and a few others), paying the tax bill itself is a burden enough. Whatever pre-payment was done the year before is largely negated by the requirement to pre-pay for next year.
We call for the requirement for Payment on Account in January to be scrapped.