I'm no butterfly expert, so can't help you there. However, Craster is a good base for birding, central to several excellent locations.
In addition to the Farne Islands, you can take a walk from Craster to Low Newton, along the coast past Dunstanbrough castle and along Embleton bay. Low Newton is a good place for waders and migrants (and also has a great pub - The Ship!). Heading north of Seahouses, the area around Bamburgh has some good areas, including Budle Bay and the sea off Stag Rock. And further north still, Lindisfarne is an excellent place to spend a day, with waders aplenty as you cross the causeway and coastline that attracts many species of seabirds.
If you head south from Craster, to Amble, there is a fantastic stretch of coastline with half a dozen or more great birding sites, including Amble braid, Hauxley NR, East Chevington, Druridge pools, Cresswell pond, down to the headland in Newbiggin which is good for seawatching.