Rectifying Past Ad-Hoc Income & Navigating New Job Tax Forms

I have been doing ad-hoc "work" since 2019 for a company, earning between £7,000-10,500 per year. Not much, but living with roommates made it not too difficult. Now, to my absolute daftness, I wasn't aware that ad-hoc work qualified as "real" work and needed to be declared (lack of education on the matter) and as such I haven't declared any income yet, regardless of me being below any taxable threshold. I intend to declare the current tax year after now learning more about this but I need to know what I should do about the previous years? Can I do a backlog without issue? Will this prompt a full-blown investigation and penalisation? Furthermore, I just signed a contract for my first full-time job and they've asked for a P45, which I of course don't have as I wasn't contractually employed. They emailed me an Employee starter form to fill out for them, I am not sure which option to select:
A: THIS IS MY FIRST JOB SINCE LAST 6 APRIL AND I HAVE NOT RECEIVED ANY TAXABLE ALLOWANCES, BENEFITS OR PENSIONS.
B: THIS IS NOW MY ONLY JOB BUT SINCE LAST 6 APRIL I HAVE HAD ANOTHER JOB, OR RECEIVED TAXABLE OR INCAPACITY BENEFIT. I DO NOT RECEIVE A STATE OR OCCUPATIONAL PENSION.
C: AS WELL AS MY NEW JOB, I HAVE ANOTHER JOB OR RECEIVE A STATE OR OCCUPATIONAL PENSION.
According to the HMRC, self-employment (or anything that isn't contractual) should be ignored for these questions, so I'm inclined to select A — however, this doesn't fall in line with the fact that I have earned roughly £11,300 this tax year which is under the threshold but, as far as I'm aware, still classed as "taxable allowance". What do I do? I want to get this resolved as soon as possible as my new employer obviously needs to use a tax code for my employment and I evidently don't have one yet! Thanks to all advice in advance.

Hi,
If you earn more than £1000 of untaxed income in a year, you need to complete a tax return for that year.
Just contact us by webchat or phone via:
Self Assessment: general enquiries
HMRC can register you for Self Assessment, and give you more advice on how to complete your returns from there.
On your employment Starter Form, select Option B, as you had untaxed income earlier in the year - selecting Option B will ensure that only a month's worth of tax free allowances is allocated against your March wage.
Thank you.