Hook64

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.