If you mean "Field Guide" as in having it in a jacket pocket to nail identifications in the field, then it is not suitable. It is a pretty bulky book, and probably better suited to the "book/map box" in the boot of your car.
For UK butterflies I would suggets David Tomlison and Rob Still's "Britain's Butterflies". It is pocketable, has lots of photographs, and is pretty up-to-date.
The other excellent option is the "British and Irish Butterflies" by Adrian M Riley. A brilliant book that also details all the sub-species, and is essential reading for the butterfly nut.
If you plan on visiting Europe ther are two excellent books-
Collins Field Guide Butterflies Britain and Europe (Tolman and the brilliant illustrations of Richard Lewington) or Butterflies of Europe by Tristan Lafranchis - loads of photos and good identification pointers.
There are lots of others that other people on here may prefer, and I am sure they will let you know
Good luck