Registering as self-employed

[CAPP0]

Scenario: Person A has been doing a low-paid job for a little while (a year or so), essentially as a casual worker, no contact, no fixed hours, paid in cash. No tax paid as they are well under the personal allowance and have no other income, and, perhaps incorrectly, no tax returns submitted (ever in their life, including when working full time as a salaried employee in the past).

They now want to step their work up a bit, and earn a bit more money/pay NI, and tax if required. Not clear yet whether this would take them over the threshold for paying tax; let's assume it does but not by much.

The question is, when registering with HMRC as self-employed, should they back-declare the time up until now when they have been working? They have records of monies earned and can prove/justify that, but as mentioned haven't hit the threshold, so to the untrained it would seem that no offence has been committed. Would it just be easier to say "I'm starting now" and/or would it be a massive ball-ache to retrospectively track back and submit (and would the Revenue have the hump about that?).

Thanks.