What they send out is a lancet, a vial - which might contain a tiny amount of EDTA or other substance to preserve the sample, an alcohol wipe, and packaging. Such things do need approval - that is, the postal people demand that it is an approved packing system so that their staff will not get infected by any spillage.
The person who receives the kit, stab their finger, squirts some blood into the vial and pops it in the post (using supplied packaging).
Several possible steps requiring standards be applied - but not at all clear whether they are required for GB to NI or NI to GB or both.
Other than the used lancet and some packaging and used wipe, everything gets returned.