THIS IS THE RIGHT ANSWER. Optobear, with all the more authoritative attempts at answering your question the correct one was about to be ignored.
The answer to your question is not that banks were lending at a loss below base rates to 'increase market share' but that they were indeed making a profit all along because ther funding cost was also below base rates. To understand you need to look at how banks finance these mortgages and understand a bit about wholesale funding: banks don't base their fixed rates on the BOE rate but on the wholesale bond market's swap rates (which is where they get their funding from); and swap rates have been lower than the base rates for most of 05 and 06 (which is arguably a result of the UK's position on the global yield curve).
Here's an article from Pimco (June 2007) that explains things much more clearly than I ever could. I find Pimco are a good reference if you want a reliable perspective and understanding of bond markets:
http://www.pimco.com/LeftNav/Global+Market...07+Bradshaw.htm
The chart below from the same article shows the spread between the STG 2 year swap rate and the average UK 2 year fixed mortgage rate until mid-2007 (blue line). It shows that the spreads have indeed been reduced, mainly because the originators' risk component of the spread became unnecessary thanks to securitisation (or 'Ponzi scheme' as Pimco like to call it at the moment).
Are the banks responsible? All you need is to look at who benefited, and the 2007 bonuses bankers paid themselves in the midst of this crisis gives you the answer. I pity the poor bank shareholder who has to read about these obscene remuneration packages while his savings are being comprehensively destroyed.