Saltney Jn-Wrexham North was singled around 1990 (partly because the new A483 route cut the line, so only a single bridge span was needed).
Half of the 10-mile single track (north of Rossett) was redoubled recently but the other half is still a constraint.
While the main Cotswold line was singled around 1973, and is now partially redoubled, further down the line the Ledbury-Shelwick Jn section was singled around 1990, with a line speed of only 65mph (was 90mph).
Worcester-Hereford therefore has a multiplicity of single line sections, and if you add the Droitwich-Stoke Works section that's 4 single line sections on a Birmingham-Hereford trip.
The Marches line (Newport-Hereford-Shrewsbury-Crewe/Chester) also came close to being singled, and it was probably the steel freight traffic to Dee Marsh that saved it.
Chester-Manchester via Northwich is another awkward route with multiple single-track sections (the route through Cheadle was always single, but the rest was singled in recent times).