I am now advising my friends to consider strongly investing and buying outside of London, and then using the rent to pay the mortgage and subsidide your London rent. This way you can build equity and still live in London. Lots are starting to see what I am saying.
I asked a mortgage broker about this three years ago and he said it would be extremely difficult (or perhaps impossible) to get a BTL mortgage if I wasn't already a homeowner.
My son's house (3 bed terraced very end of a tube line cost £325k about a year ago - not cheap but not impossible if you do what we, my parents and their parents did - buy before you have children, put off babies, have two full time salaries)
Again, like many comments above, I'm afraid this is naive.
I don't know how old you are, but my parents were able to buy before having children because they bought in their early twenties. (In fact, my dad bought his first house at 24 on a single salary and he and my mum sold it before they turned 30 because they felt it was too small to have two children in. The same house is currently on the market for £600,000 and a season ticket into London from that town costs over £4,000.) The problem young people have now is that they're having to scrimp and save for so long to get on the property ladder that by the time they can afford to get on the property ladder with a starter home, they're at the age when they want to be having children, and the starter home that would have done for their parents isn't really big enough. They can only afford to buy a starter home when their parents at the same age might have moved on to their second or even third home with more space for their growing family.
It might be taking people ten years longer to get on the property ladder than their parents did, but women aren't fertile for ten years longer than their mothers were.