Nothing wrong with the postal HIV blood tests - they all usually go to The Doctors Laboratory in London (TDL) which nearly all the private STD clinics use.
If you are having trouble getting enough blood then try this. Warm up your thumb or index finger by either putting it in hot water, or sitting on it for a few minutes. Dry and clean with alcohol wipe. Stab yourself with the little pin prick thing they give you at the very top of your finger or thumb, push upwards to force the blood out of the hole and into the tube.