Invited to a meeting where I will receive a postdoc offer, what should I discuss/prepare the meeting?
Read the agreement between your university and the union (presumably UCU) if applicable.  It's boring.  This defines what you can request.
Read benefits information on the HR website.
Read visa rules on the government website.
Figure out your preferred start date.
Estimate your relocation costs.
Find out what local taxes are.
Find out local housing costs from a regional housing website.  (In the UK they are often unreasonably high.)
Make a list of questions to ask.  Questions should demonstrate that you are genuinely interested in the position. They are not negotiation.
Reread the job ad.  Some places will try to offer you a job you didn't apply for.

For a postdoc, it is not called a startup package.  If you ask for &quot;startup&quot;, people will think you don't understand what a postdoc is.
Unless you are an experienced negotiator, do not try to negotiate during a meeting.  Negotiate by email.  Get everything in writing.