I think the current American IR books are the best for serious IFR stuff.
Trevor Thom book 5 is what you are supposed to read for the IMC Rating but I found that book (back in 2001) to be very poorly written and probably the work of several people, stitched together. It was also barely relevant to modern IFR which is done wholly with GPS, backed up by VOR/DME when available.
In "UK Class G informal IFR" one is not going to be flying holds or anything else too fancy. One cannot have legit IFR rights on an NPPL (i.e. all types of approaches into Class D) but from a pure safety POV what is highly desirable is flying an ILS, which is hardly difficult. A sim, and some ground instruction, is the way to get this started.
Radio nav is 100% usable for normal VFR flight too; I have been doing that on every trip since the day after I got my PPL.