Salaries (of any job - not just academic) are usually due to a number of reasons -
1) The demand for that job
2) the number of applicants
3) The value that the employer puts to that job / person
4) How much the employer can afford (sometimes but not always related to 3 above)
5) What the employer can get away with
In case of UK, I think (5) plays a more prominent role than the other 4 reasons. The annual salary for lecturers tends to start at £ 31,000 with very low increments. After taxes, and contributions to insurance etc, this yields around £ 1800 per month, which admittedly is a pretty miserly sum when one considers that at least half of this is spent on rent.
1) Demand for a lecturers job - is fairly moderate / low. There tend to be a few jobs advertised - but most of them are the result of disgruntled lecturers leaving. Very few jobs are being created due to additional departments or growing numbers of students. Unlike countries like China and India where going to college is the norm for people of all classes (and hence people save for college since the child is a toddler), it is not so in UK. Education is not a priority for the average UK citizen. This simply lowers the number of students further.
2) Number of applicants -
Actually, due to the above stated reason, there are a rather limited number of British citizens available for faculty positions. However, this tends to be compensated by educated people from EU, and sometimes from Asia. While British teachers consider it their right to be paid fair wages, and demand them, this is not always so for their EU and Asian counterparts. The result supports reason (5).
3) The value that the employer puts to that job / person
Is quiet high. Education is a big business in the UK and lecturers are among the most important cogs in this apparatus. However, one always needs to assert this importance to prove it in the UK (not just in academic, but in many other professions too except perhaps for banking, financial services, real estate and politics).
Student feedback is taken regularly and published openly (however, only around 20-30% students actually respond to these official surveys thus limiting the visibility of good lecturers in the larger scope of education)
4) How much the employer can afford -
Unlike many other sectors where employers genuinely cannot afford to pay more, universities are not so. They earn rather well, and the fees paid by EU/UK as well as overseas students tend to be to the tune of £ 8,000-£ 11,000 annually in an ordinary university. At UCL and Cass, it is even higher and to the tune of £ 15,000-18,000.
These tuition fees cannot be directly compared to tuition fees in USA and other countries as actual classroom lecture hours in the UK in most universities are lower than their counterparts in USA - as low as 12-16 hours per week for a Masters level course, and just 20 hours for under graduates per student per week.
Also, Universities tend to pay far lower rates for rents, gas and electricity than their business house counterparts.
5) What the employer can get away with-
Sadly (for lecturers - and hence students, as good lecturers tend to leave) and happily (for deans, and upper level management), universities in UK have been able to get away with low salaries. The fact that the lowest median wage in the UK is even lower makes it that much easier for universities to escape scot free. The fact that the number of British students attending classes in UK is so low, means that this is not an issue to the British voter currently. You will recollect that none of the campaigns last year said that they would try to improve the education system for British people.
And hence, UK faculty salaries are so low.