Portsmout - I haven't been on one, but looked this up on a search engine, "DuckDuckGo" which came straight up with the website of the British Library and it's tours.
They are only done in certain months of the year and usually on most days of the week, but not every day from when I clicked on the booking button, Tuesday was missing.
It's £10 for the tour and if under 18 years old, £8.
When I went to the British Library, I wanted to look up the microfiche of old British newspapers - I wasn't allowed in as I had no ID on me, they wouldn't let me in without, so if you have a driving licence or passport take these, with a bank statement, or council tax bill as well. They insisted on me having 2 forms of ID - so I never got any further in, which I was really annoyed about.
I'm sorry I don't know. I just popped in to kill time before my train left King's Cross.
The British Library has fabulous exhibits and permanent collections. It is free to enter but there may be a cost to see a special exhibit, as there was with the recent one on Captain Cook. Best to check on line for times and any extra events.
To check tours go to the main British Library website, go to the visit tab and then go to the tours section. They include a calendar for tour dates and upcoming events. The library itself is free to enter as is the "treasures of the British Library" room.
Vincent D - you'd want to tour as this library has an enormous collection of historical items, such as the Magna Carta for instance, so do some research on it and learn :)
Unable to answer the tour question however it is free to enter
Why would you want to tour a library?
the library consists of extensive bookshelves.
i believe the only way is to:
Check the catalogue at home on the internet.
visit the library.
Read the book.
Other than this i don't understand the question.
Get quick answers from British Library staff and past visitors.