A finger prick blood test collects a very small amount of blood. It can't collect enough blood to say for certain whether you have HIV or syphilis. Instead the result will tell you if it's reactive or negative. Reactive means that you may be infected. You will need to have this confirmed with a blood test taken by a clinician at a face-to-face appointment.
This is standard practice in the UK and applies to all currently available at-home HIV and syphilis tests. A GP will talk you through the next steps if this happens.