I think there are many factors but the general right-wing bias in the UK media has a huge influence on the UK's political culture and the orientation of the average person. Left-wing politics could counter this but is still refracted mainly through the Labour party in most parts of the UK.
There needs to be a mainstream political party sited to the left of Labour in England. It needs to have a consistent programme of policies, an activist base (and be based on these activists not on personalities at the top of the party like the SWP and both Respect projects have been) and a proper branding and professional organisation.
The reason I say 'mainstream party' is so that it can get a presence in the media and get the party name out there so it has a recognition factor (how many people in Wales have heard of Plaid Cymru, or even how many people in Scotland have heard of the SSP), the policy programme can still be radical as long as the campaigning priorities are sensible. Most people aren't that interested in the Lebanon conflict but are very interested in affordable housing. It would still be essential to be on the Lebanon demonstrations but the front page of your newspaper should carry national issues instead- if you choose to have a paper that is, I personally think they are a waste of money and the cash should be spent leafleting entire estates.
You need to get to a point where people in the workplace will describe themselves as 'i'm PARTY NAME i am', how many people can identify with 'the Left List' or even 'Respect', certainly not enough to have an influence on a national level.
Within this mainstream party i'll concede that some would be reformists or social democrats on the right of the party, but there would also be a place for revolutionary positions on the left of the party (even as an eventual aim of the party).
If the left retreats from the mainstream into bickering far-left factions then the playing field will be abandoned for the likes of the BNP to walk into the London Assembly.
You could easily dispute my point and carry on with your left lists and what not, but the point remains that England/the UK needs a party to the left of Labour (even to the left of Old Labour) that is capable of winning elections and having councillors, MP's and Assembly members. That is the only way of making sure a left voice is heard in the UK political sphere. Eventually they will stop inviting the leftists onto the political tv programmes and debates because the popular demand for them is disappearing.
I would love a full debate to open up on this because I think I have more points to make.