The main reason that we recommend a venous sample or even remove the finger-prick option in the checkout is simply because of the volume of blood required.
If you buy a test (or a combination of tests) with a lot of blood markers in it you will find that you do not have the option to take it with a finger-prick sample. This is because to get enough blood for testing you would have to fill at least 3 vials of blood which, in our experience leads to a very high error rate and a poor customer experience. Your blood will start to clot after you have made the initial puncture which, in time, obstructs blood flow and often leads to a lot of squeezing and "milking" of the finger to get enough blood. This can damage blood cells and make the sample unusable. You may not even be able to fill the tubes to the required amount which means that the laboratory will report back that it had "insufficient sample" to analyse.
This is why we have built a network of partner clinics to enable you to have a venous sample taken quickly and affordably in a clinic or pharmacy near you.