UK Postdoc - Is salary negotiation even possible?
There is no reason why a post doc cannot be appointed at a higher point on the spine if there is money there, that said, rarely is there money available. The number of people doing multiple post docs in the UK is smaller than in the US, and they usually stay at the same school. That means they often can be appointed to a higher grade. Therefore grants for post docs usually budget for the lowest salary and normal progression. A school or university funded postion would have more flexibility, but the head of school is not going to authorize more money from their budget.

I would not bring up the salary until after an offer is made. Once an offer is made, it cannot really hurt to say "hey, I deserve to be at the top of the scale since ...". They will then laugh and say you are right, but that is all they can offer.