It's interesting to note that that the three biggest parties capitalise on their leading positions by garnering far more air time than any of the minor parties, thus reinforcing their position. This creates a strong barrier to entry for any other party or independent politician trying to get seats in Parliament.
Quite apart from the power of their brands, the leading parties also attract more funding than the lesser parties, which enables them to entrench their power base. They have money to spend on a much more powerful campaign.
Occasionally, independents do get seats in Parliament, but usually they already have some sort of brand presence in the media. For example Martin Bell, with his distinctive white suit, was a BBC correspondent.
These political barriers to entry are another way that democracy is skewed from the ideal. Interactive Democracy would give voters more power, allow new ideas to emerge and foster wider debate of individual policies. However, it still relies on party politics to form a credible government and does little to undermine the three main parties as political brands.
(Please see Porter's 5 Forces as another perspective on the power balance within politics.)