At the British Library also it is prohibited to photocopy from older books.
They do, however, have a gentler way to copy and will do this by efforts of the Library staff.
There are a number of documents relating to Portugal there, not just documents relating to Beresford time, but also Schaumberg Lippe, and some books from Portuguese writers about those times.
You can access the British Library catalogue online at http://catalogue.bl.uk/
I believe you can also order copies of pages online (assuming you know which pages from looking in Lisbon) and have them delivered to you in Portugal.
If that does not work let me know what you need most and I will try to get some copies next time I visit. (I am trying to study the [Portuguese Infantry too)