Dear yla92,
Apart from being in London, you could be telling my story. I am a 43 year web developer, I have suffered from major depression for most of my life and I have often thought about killing myself. For far too many years, I was utterly miserable and assumed that I was just sad the way some people are blonde and some people are tall; it was just part of who I was and could not be changed. It took a long time for me to realise and accept that I was not inherently fucked but that I was suffering from a condition that distorted my perceptions of myself and that made me want to kill myself.
Part of what made it hard for me to seek help was that I did not think I deserved it. We were well off when I was growing up (both my parents are doctors), I have all my limbs, I am white, straight and male and not subject to persecution on grounds of race, gender or sexuality, I do not suffer from a physical condition or disease. I felt that I did not have a right to help because of the advantages I had had growing up and what right did I have to feel like crap when I objectively had it so good? I mention this because you mention your opportunities in life. This does not invalidate what you feel in the slightest.
I think that talking to someone -- Samaritans or someone else -- is crucial. I wish I had sought help earlier than I did. I would have wasted so much less time. It took me a couple of tries to find a therapist who worked for me but it has helped no end.
How do you break it to your boss and colleagues? Who says you have to? If you had cancer, kidney failure or one of any number of physical conditions that required time off work for treatment, would you be required to tell your co-workers about it or would be a matter of utter confidentiality? Would your reputation be marred if you needed to take time off for dialysis? I am well aware of the stigma around mental health and depression but that should not be a barrier to getting help. It is possible to get help and still be respected by those around you. (http://en.wikinews.org/wiki/West_Australian_Premier_Geoff_Ga...).
If there are people at work who you can confide in, I would suggest reaching out to them. If you can afford to take time off, I would suggest considering it.
Know that you are not alone. As you suggested, there does seem to be a correlation between developers and depression. We are many.
This sounds like a truly dark time for you. It will get better. Don't do anything permanent to fix what is a transient situation.
Kia kaha, brother. Stand strong.