Thank you very much for your very helpful reply and advice. I just found out that Aberdeen is asking for an A in A2 Chemistry. I have only a B, but an A for AS and an A at GCSE. Wouldn't have a chance??
Requirements for graduates are available here: link.
They sometimes make exceptions, but there has to be a good reason. In some circumstances, applicants can be given a conditional offer, bundled with an invitation to attend summer school course in chemistry.
The best choice would be to contact the admissions staff directly (call them) and ask for advice.