Are these official UKMT past papers?

No. RubricMark practice is aligned to UKMT challenge formats and timings for timed sits.

UKMT Mathematical Challenge Practice | RubricMark

UKMT Junior, Intermediate, and Senior Mathematical Challenge practice: timed 25-question papers online..

Study Lab and Practice Hub

RubricMark is a personal learning companion from Year 5 through university. Study Lab offers a curriculum map, outlines, cheat sheets, and daily articles. Practice Hub covers HAST, Selective, OC, NAPLAN, IB, and senior exams with timed mocks and rubric feedback. Practice Hub covers IELTS, PTE, and JLPT with personalised practice and study books.