Hi Andrew
Welcome to the Forums.
Not clear from your posting as to what previous experience you have of the workplace, but even if it might only have been doing "work experience" when at school or a "Saturday job", think about what you learned that gives you transferable skills that you could apply in a new job - both what could have been better (!) and what was done well.
It should go without saying but......"DO YOUR HOMEWORK" - organisation and each of the people you are touring the facility with. As example the H&S and E person might have a background in something else and it would help if you have an idea of where THEY are on their learning curve + whether they are based at the site(s) where you would be working or at some regional or (inter)national office.
Assuming that the organisation is large enough to have a website, then you can probably find out a lot about what they SAY their views on H&S and E are + there may be policies and certifications. May be those will tell you a bit about their approach.
Most importantly, the central element of this interview is a site tour, so thinking about what you expect to see and have some QQ already in your mind - "how does this work?" "what are the risks?" "what do you do to reduce those risks?"
....which types of QQ will be relevant to various parts of the facility.
So, speculating (!) I can guess that outside there will lots of road (and possibly rail or marine) vehicle movements (including in relation to the offsite logistics), + possibly storage of bulk chemicals, some of which might be toxic and/or flammable. May be COMAH is relevant, perhaps not.
Inside there will be processes which depend on what is being made. So work at height, dangerous machinery, mixing of hazardous materials, noise, vibration etc etc.
Depending on what is being made there may be HSE/Environment Agency guidance or industry standards that will help you get an idea of the key challenges that are relevant.
Be prepared to admit your ignorance - it's a first job in H&S and E so the interviewers should not expect you to know all there is to know. If they do, then they shouldn't have invited you to attend this tour.
Keep your eyes (and other senses) peeled - you never know, with a fresh pair of eyes you might see something clearly problematic which has just become accepted "custom and practice" for those who have worked in the facility (INCLUDING any H&S and E people) for years.
When I worked for the HSE I used to be regularly surprised when some organisation would volunteer to be a guinea pig for a group of Inspectors - getting one or two at a time is a challenge, inviting 20 HSE Inspectors to site at the same time is brave!
So, on one such visit we were touring a car manufacturing plant. We were prewarned that a robot accident had happened and was still under investigation but the visit was going ahead anyway.
After a brief session in an office, the manager invited us to walk around the plant and gave us cart blanche to look at whatever we wanted and to ask any of the workers any questions we had - VERY brave!
The plant must have spent a fortune on machinery guarding. Area enclosures, time delay interlocks ete etc around the robots doing most of the work.
But, I just touched the perimeter guarding at a corner and it gave way. The same manager saw this happen and sheepishly commented "I see you have found where the operatives find a way in when there is a problem".
I didn't ask whether there was any connection between this and the accident that had happened - that one down to the local Inspectors doing their investigation. May be management knew about the weakness in the guarding that I found, may be this came as news even during our tour.
If you see something that appears to you to have room for improvement, ask, but don't judge! There may be good reasons for what is being done.
So, be curious and good luck, Peter