I couldn't negotiate my postdoc salary during HR meeting, should I try days later? (I haven't received the formal offer yet)
In the UK, it is not common for postdocs and entry-level lecturers to negotiate their salaries. This is different from the customs in the USA and some EU countries.
In the UK, the starting salary of a postdoc or a lecturer would normally be set up by HR in consultation with postdoc's PI or lecturer's HoD. As a postdoc, if you want a better offer, you have the following options:

Provide additional evidence of your relevant experience. Note that an experience of course design and &quot;helping professors with teaching&quot; may not be considered relevant for a research-focused postdoc position. And even if you have relevant experience, it may not be considered sufficient by the PI or HR.
Tell them that you can not accept the offer unless they raise it. This is a risky move, which can cost you an offer. Bear in mind that HRs are not really interested in hiring you, their goal is to ensure the hiring process goes smoothly and according to the rules. They may approve your claim, provided there is a budget for a raise and the PI/HoD supports it. Or they can send you a polite email starting from &quot;We are sorry to hear that you declined our offer&quot; and offer the job to the next candidate.

Before you proceed, note that if your postdoc salary comes from a grant, there may be a firm cap on how much the University can offer you from the limited grant budget. You may want to ask your PI if there is even any room for negotiation before discussing it with the HR.