I feel weird posting in a sticky last posted in 2017, but there is a lot of misinformation ITT. You would need to study for a 4 year degree in the US or study for 2 years in the US after completing your degree in the UK.
If you take the second option, you could have a UK degree in anything, but you would have an advantage if it is in a related field. If you already have a degree in something to do with biology or chemistry, particularly from a top British school, then you would have a good chance, as you would have an advantage in the classes in the US against beginning college students and would have a strong academic background to score high on the MCAT.
It might also be possible to study for an undergraduate degree in Canada, which might be less expensive and similar to the UK system.
Now the published requirements seem minimal, but are not real requirements. The main thing you need is high GPA and high MCAT score. They adjust GPA some based on school and major, but not enough, so it is more difficult to get in from top US schools than average ones.If you decide to do this, you will have to go study in the US with no degree of getting into medical school. The statistic that half of students applying get in is misleading, as many don't apply because they know they don't have the numbers to get in.
It is extremely expensive. For US students that isn't such a big issue. They can get low interest government loans and high interest bank loans betting they will make enough to pay them off. Also, half of US students have a parent who is an MD, and they usually make enough to easily pay the tuition and maybe donations to the medical school they went to. In general, most of the US students probably have parents who can pay the fees.
They need doctors everywhere, and it is much easier if to get into a black medical school or instate admission from a rural area or in a backwards state. Most of the students in those categories had very poor high school educations. As a foreigner you won't be instate anywhere and will need really good academics. If you were a US citizen and did not have the hooks of either money or connections or being from a needed demographic group, you would be in almost the same situation. So basically you need to be very strong academically and have a lot of money, and still you may not get in after doing your US premed classes.
Yes, they do accept US students with other majors who just take the minimum premed classes, but most applicants have taken many advanced biology and chemistry classes. The MCAT does not explicitly cover that advanced material, but you need a really high score on it.
Most students at top US medical schools had extremely high GPAs and MCAT scores and demanding related majors at top schools. However, taking the tough undergraduate program risks not getting in at all if you don't get the grades.
So if you have a really strong related academic background and lots of money, then it may be worth a shot.