How to negotiate the salary for a UK faculty position?
Salary is generally not very negotiable in a UK university. Further, the benefits of a higher salary are less pronounced in the UK than in the US. Pay in the UK is defined by a universal salary scale/spine. There is a maximum point on the spine you can achieve through normal progression. Progressing pass the standard maximum is extremely rare.

I have never heard of a department hiring beyond the standard maximum. What this means is that if you start at the standard maximum (the highest point on the scale obtainable through standard progression), you will make 10k more  then a colleague who starts at the bottom. After 3 years, the pay gap will be only 5k (still a 10% raise) since the person who started at the bottom progressed and you did not. If you both get promoted at that point, you will both start at the bottom of the next tier. In other words your negotiation to move from the bottom to the top was worth 20k over the course of your entire career. If promotion takes longer, the difference maxes out at around 36 k (depending on the specific spine points).

Many department offer the lowest spine point. It is often, but not always, possible to negotiate up a few steps based on years of experience. In my experience as an American who worked in the UK, Brits find it insulting to negotiate a startup package and salary. It seemed most Brits took what they were offered (or maybe an extra step or two) while international candidates asked for the top of the range since the salaries are much lower then they expected. The international candidates are then internally laughed at for being cheeky yanks, but often paid more then their British counterparts.