What I did was to annotate the documents with the GBP equivalent because I had a lot of financial documents and then I put the source for example, xe.com in brackets.
+1
I also created two tables with all figures (1 in original and another in GBP)
Took snapshot of original document and pasted it next to table. created a link to original file on drive.
Also gave reference link to XE.com website for forex rate.