The advice that you have been given to seek out a market niche in which to operate rather than compete in a crowded market place on an undifferentiated basis is sound. However, I would just make two observations on what you have said above.
- The niches you have identified are still very broad.
- You are identifying niches more relevant to potential customers already in the industry rather than to the majority of the market.
Remember that customers make up markets whilst suppliers make up an industry. I don't think that the majority of customers that make up the market for web development think in terms of "AJAX developer", "Drupal developer" or "PHP programmer". If you want to do contract work with marketing agencies this might help (a little because it is still very vague), but I think that this kind of distinction is lost on most potential customers you could be targeting (e.g. small business owners or private individuals setting up their first website), so presenting yourself in those terms will not really help.
I think that the niches that you need to address are more like "web developer for tradesman looking for low maintenance on line source of leads in the north west of England" or "web developer for professional service firms in Leeds". The trick is to narrowly define your niche to eliminate as much direct competition as possible whilst leaving it wide enough to contain enough business to generate the living you deserve. If the niche is too small or you become more ambitious, once you have conquered one niche there is nothing stopping you from tackling an adjacent niche. Over time, you could take over the world by spreading your domination from one niche to another to another, until you have grown your business to a size that works for you. If competition within the niche you have defined is still to fierce, resegment the market again.
Ultimately, you will need to find a niche that matches your strengths and interests so nobody can tell you which one would be right for you. However, I hope this has helped you on your way to at least choosing a niche to try out.