A BSc is a good thing as it is likely to be beneficial in the future - if an employer is considering to equal people but you have a BSc on top it may lead to you getting a job. It’ll also allow you to learn something you are are interested in for a year which may be more research based or hospital based. UCL do some really specific BSc for med students which allow your interests to be explored and it may be beneficial to choosing a speciality if you have some extra background knowledge.
But a BSc isn’t necessary and it is an extra year of study accruing more debt (essentially its invisible debt where you’ll be paying the same amount back yearly - something like £300 - but for a bit longer rather than paying more back each year). Medicine is a long enough degree at 5yrs so why bother with another year of study as you’ll already have everything you need to be a doctor without intercalating?
Which intercalations does each uni offer and how many different types are there? There could be a couple of really interesting BScs at one uni but another could have a list of 20 which would allow you choice if your interests change during your degree.
When deciding try to pick the ones where you can imagine yourself living and studying for 5-6yrs - which has the nicest atmosphere, which has better recommendations from current/past students - speak to students at open days and look online at student satisfaction. Do you prefer lots of theory or would you like practical work early on? Which hospitals would you like to work in - UCL has Great Ormond Street (which will be very competitive to get into), Kings College has placements at Princess Royal University Hospital which is on the outskirts of London - expensive to travel here daily for 12wks but teaching is good as the hospital is less crowded with med students compared to central London.
Which has the best nightlife and societies? Which has the nicest location/campus/accommodation? London is expensive - which has cheaper accommodation, which isn’t far from halls (saving money on transport), which is in a nicer (and maybe safer) area?
How many students per year does each uni have - would you prefer to be in a group of 400 students where it’s more anonymous and you don’t know many people or a smaller med school where people know each other and lecturers know you (this can be good or bad!!)
Finally, after deciding on your favourite (I’d suggest a pro and con table for each uni or one big table comparing each uni on certain points e.g location, teaching style, student satisfaction, etc.) try to find an insurance with the same or preferably lower entry requirements.
Good luck deciding! There are really good unis and I don’t think you’d regret choosing one over another too much but I know how hard this decision can be!