Looking at the packaging, it looks like the underlying company is Thiva. They are run be reputable folks.
If I am having "private" tests done, I tend to use Monitor My Health (MMH) who are very reasonable, and is a commercial element run out of the NHS lab at Exeter Hospital, so very likely acceptable to your GP, should you want to discuss your results.
MMH are quick, efficient and helpful if you have any queries.
To be clear, I have absolutely nothing against Thriva (the DCUK offering), but haven't used them myself.