Insulation for outhouse/garden office advice

Howdy all. In the middle of building a 4.8x3m outhouse. Framing is from 4x2’s, actual width is 95mm so will insulate walls with 90mm PIR. Will also insulate cold roof with 90mm PIR to allow for 50mm air flow gap (built from 6x2’s).

I plan on building a partition wall so that approx 1/3 of the space (1.5x3m) is an office. Wondering if it’s worth insulating the floor and this partition wall for the office space? The base is 100mm concrete slab.

Also am planning on cladding inside with 12mm ply. Am getting conflicted advice about whether to install vapour barrier for plywood?

For context, I’m based in south Essex so doesn’t get particularly chilly in the winters!

Any advice greatly appreciated - cheers!