Rough Guides' "First-Time Around the World" is pretty much exactly the book you're describing — all practicalities (routes, budgets, visas, insurance, packing, staying safe) rather than destination inspiration. "The Gap-Year Guidebook" is also worth a look: UK-specific, updated annually, and covers working/volunteering/travel options plus the boring-but-vital money and insurance stuff.
Two free things worth pointing her at as well: the FCDO Travel Aware pages on gov.uk for country-by-country safety and entry requirements, and a fee-free travel card (Wise, Monzo or similar) sorted well before she leaves — that alone saves a surprising amount over a year.
One thing I'd gently plant with her: these years tend to go much better with one or two fixed "anchors" booked — a seasonal job, a course, a volunteer placement — and the pure travel arranged around them. It gives the year a spine, makes the budget predictable, and the anchors are where you actually make friends; endless free-floating travel sounds dreamy but gets lonely and expensive faster than most 18 year olds expect. And if she does look at any kind of organised placement, get her to compare two or three providers on what's actually included per week, and check whatever she books has proper financial protection for the money she pays over — same as you'd expect from a package holiday.