Current Imperial final year here... some 6 years ago, I had offers from Birmingham and Southampton too and was undecided between So'ton and Imperial.
My thoughts at the time was that I preferred a 5 year degree and the people at Imperial taking me around were quite arrogant and I didn't particularly like it... everyone thought I was mad for considering So'ton over Imperial but long story... I ended up at Imperial anyway. 5 and a half years down the line now... I have to say the intercalated degree is A LOT of fun and time flies in medical school. You all probably think 6 years is long now but it goes in no time at all and you start wishing you had longer in uni. So the intercalated year is great for that, it's also really laid back, and adds points for any job applications later on. And if science isn't your thing you can always choose something like medical humanities for your project or do a BSc in management or global health.
Socially, Imperial has always left a little bit to be desired (but I'm not convinced Birmingham with their absurd A* requirement will be socially any more upbeat within medicine - if you had said Leeds, Sheffield... etc) but it really is what you make of it. You will find some of your peers to be scarily padding their CVs at every possible opportunity and it can become a little bit of a pressure cooker at times - which isn't the case in many other medical schools. But hang with the right group and you should be ok. Also, London versus Birmingham as a city... you have to be mad to compare. You're going to be in South Kensington for your first two years, stone's throw from Hyde Park, stunning area, Central London is really close by...
Also, the male to female ratio does suck, and inevitably will have some impact on your social life either way. But you are in London, so it really is what you make of it, you just need to try more to have the same smashing experience! I have enjoyed my almost 6 years here... though by the time you guys start here, I'll be off doctoring