It depends on what you want to do. As most people have said, doing a PhD is probably the best way to attain the highest paid jobs but it absolutely isn't an easy option and absolutely doesn't guarantee career success. Think the next 3-4 years of your life earning £14,000 to £20,000 a year (if you're lucky) if you can even get onto a fully funded PhD position which are highly competitive. PhDs aren't easy either, very long hours and to stick through it you have to be very resilient and passionate about what you are doing. After a PhD the starting salary for most jobs is £35,000-£40,000 a year at the moment for lab jobs such as post-docs or research scientists in the pharmaceutical industry. There are other careers such as Medical Science Liaisons as mentioned above, these have starting salaries of about £50,000-£65,000 but to get into being a MSL is really difficult and you need a high understanding of a disease area eg. oncology and most jobs ask for experience in the role which is impossible to get if you can't get a job. So you'd probably have to do a PhD then postdoc or PhD then few years of industry experience.
Honestly if you are looking to earn money straight from an undergraduate degree (and you want to stay in science), you'd be best applying for pharmaceutical company graduate schemes or direct roles in companies. These are also highly competitive but they will immediately pay about £20,000-£30,000 a year depending on the company. The progression through your career will be slower and harder than if you'd done a PhD and you will probably end up earning less per year than somebody with a PhD but it's the best option if you want a decent salary now and you would still earn a decent amount in the future. There are obviously careers in finance or business which would pay more than being a scientist (both immediately and in the long term).