The recruitment process is essentially 3 stages: pre-interview, interview, hiring. The HR department is almost exclusively concerned about your legal right to work in the UK. I have never heard of a case where HR overruled a school/department about a candidate meeting specific eligibility requirements. With that in mind, at the pre-interview stage, HR takes your word on your ability to legally work in the UK. At the interview stage, HR (or a representative of HR) will collect whatever documentation is required to establish your eligibility (e.g., passport or visa). They do this because legally they must see the original documentation. At the hiring point, they might ask for copies of your credentials. As there is no legal requirement for them to see originals they can do this however they want.
I am a US citizen and when I was hired in the UK, they took a photocopy of my passport during my interview. There was some emailing of copies of my credentials after the offer was made but before the contract was offered. As I mentioned in this answer I do not have a copy of my diploma. On my first day of work, they asked for a copy of my diploma. I told them it was on a container ship and would be here in 8-12 weeks, but I could show them my transcript. They were fine with that.