There are some books that emphasize coins of Roman Britain.
"Roman Coinage in Britain," by P.J. Casey, Shire Archaeology, which was very inexpensive ($3.50 used in the US on
Amazon, less in England)
"Coinage in Roman Britain", by Richard Reece. Seaby Books. (Amazingly, there is now one copy at $2.67 in the US on Amazon--buy it! It's a $20 book)
"The Coinage of Roman Britain", by Gilbert Askew, is too old and has no photos, only line drawings.
Casey also wrote, "Understanding Ancient Coins" which is an introduction with more about Britain than anywhere else. (one copy at $4.99)
"Coins and the Archaeologist" edited by John Casey and Richard Reece has articles emphasizing finds in Roman Britain and the best (by far, and well-illustrated) article on ancient counterfeits. (none below $66 in the US, but one in England cheaper.)