To qualify as a doctor in the UK, the usual route (after meeting the relevant entry standards) is to study for 5 years to obtain a degree in medicine, followed by a 2-year foundation course of general training and then 3 years of specialist training.
However entrants to the profession who've already got a 2.1 or a 1st class honours degree in a science subject may be eligible for an accelerated degree course, lasting only 4 year instead of 5.
Medicine is the second hardest degree course to get onto in the UK (after Veterinary Science), with fierce competition for places.
Further, medical schools often expect entrants to have had some relevant work experience (either paid or unpaid) prior to starting their studies.
So, while your existing degree might be of some assistance to you (if it's a 1st or 2.1 in a science subject), it's not actually going to make a vast amount of difference to the time taken to fully qualify as a doctor (only cutting the time down from 10 years to 9).