i have 15 x 5.5 rims on my ke20 but the engineer also made up a new tyre placard sticker and measured the track again.
your choice of rims simply have to "be deemed suitable by the signatory" (being engineer) within the limits. then you get adjustment of records form for the RTA, blue slip, engineers report and take them to the RTA, and get a new rego sticker printed to relate it to the engineer report.