@TheTruthICantSay - those cities do have a lot going for them (I regularly visit both Birmingham and Bristol, so I know them quite well), but they're not a patch on London when it comes to dynamism and excitement.
If you want to go to the theatre - in London, it's a case of 'what kind of thing do I want to see?' and you'll have loads of options. In B'ham, it's a case of 'what's on at the Hippodrome or the Alex?', and if neither of those are your thing, that's it. Same for museums - London has the Nat History, Transport, Imperial War, British, V&A, Science museums and so, so, so many more (both big and small). Birmingham has a couple of smaller ones and one in the Town Hall, but once you've seen them once, you're done. You could keep visiting the V&A every year of your life and see something new each time.
Then London has all the other attractions - Houses of Parliament, London Eye, Madam Tussauds, Tower of London, Royal Palaces etc etc (not all of these are to everyone's taste, but there's something for everyone, and something new every time you go).
And you can walk miles and see loads of interesting stuff - the South Bank, The City, the parks and open spaces, the canal network etc etc. A walking tour of Birmingham wouldn't take that long...
And the viewpoints - the Shard, the Sky Garden, the cable car, the new skyscraper whose name escapes me - all great for having a view over London and its unique skyline. There's nothing like that elsewhere in the UK.
Then there's the variety in the food - you can get ANYTHING in London. Any cuisine, at any degree of formality/poshness. Birmingham has great options, for sure, but it doesn't have the depth and variety.
And then there's all the different areas/districts - Brixton, Battersea, Marylebone, Wimbledon, Shoreditch... all of which are different and worthy of a mooch around. You don't get that diversity in Birmingham.
We have other great cities in the UK, no doubt about it - but there is SO much more to London.