For the purpose of applying for Portuguese citizenship, birth certificate are valid for 6 months and Police Certificates are valid for 3 months
UK birth certificate can be ordered online here
UK Police Certificates can be ordered online here
Both documents, before they are submitted to the Portuguese government need to get an Apostille, which in the UK is also called legalization.
UK Apostille or legalization can be ordered from the government
or via a private company (which usually costs a bit more but provides better service)
What’s An Apostille?