Possibility of negotiating a better contract if I am offered an asst prof position?
In the UK, there is typically much less negotiation of entry-level academic contracts, compared to the US market, or general stories of how it was in past. Full professors still can negotiate their contracts, but assuming that it is your first permanent post, I would be surprised if the University would engage in a negotiation with you. Typically, the universities in the UK will match your current academic salary or offer you to start one spine point above it. In exceptional circumstances they can offer two spine points, but "I am making a sacrifice here for you guys" is hardly going to count as a good reason. You are expected to compare your options, and if you indeed have a better deal elsewhere, you'd better take it. Comparing their offer with hypothetical offers you could have got from some industry is likely not going to work.

But before all this becomes a real problem, you need to get their offer first. And to get there you typically would need to pass through the interview and convince the panel that you understand what you are getting into and you are genuinely interested in the opportunity as it is offered to you. If you are less than enthusiastic about this particular university and/or the conditions they offer, I would suggest to try and discuss this during the interview, so that the panel can make an informed decision whether to offer this job to you (and perhaps agree on your conditions) or another candidate. If you confirm your interest in the position during the interview and try to re-negotiate the conditions afterwards, it is quite likely that after some discussion the post will be unfilled and the position has to be re-advertised, which is really quite frustrating to everyone involved.