Legalise only what was asked for
The most common way to lose a month on this route is to over-prepare. Somebody asks for one certified record and a whole folder goes off instead, on the theory that more evidence must be safer. On a consular route it is not: each extra paper is another fee, another envelope and another wait, all running at the same unhurried pace.
Why the route is long in the first place
Guinea-Bissau has not joined the Hague Apostille Convention, so no single certificate can speak for a whole file. Embassy legalisation is required instead, which means the receiving side satisfies itself directly about the British signatures in front of it. Length is the price of that arrangement, and it is exactly why the size of your pile matters more here than it would elsewhere.
Getting each listed paper into shape
For every item on the list, work out where that paper is coming from. Sunderland Registration Service issues certified copies of births, marriages and deaths recorded in the city, so a civil record can be reordered instead of risked in the post. Do that once per listed item and no more, and the pile that travels is the smallest one that can do the job.