Tax on foreign based income

I have an Engineering/Specialist in Humanities/International Services Visa and am teaching at an Eikaiwa. I’ve been offered a part time job as a test examiner, a job I would perform remotely, and for which I would be paid in GBP.

Until now, my Japanese employer completed tax forms/withheld taxes from my income.

How would I go about reporting this foreign income, and how can I ensure I am paying the taxes due? Would this be additional income that I report on the forms my employers has been completing on my behalf?

Thank you in advance for your responses!