On the Ordnance Survey and Walk Northern Ireland websites: 100 favourite British walks ('The best of Britain and NI as voted for by you'*), as featured on ITV in January 2018. The GB ones include links to the routes on OS mapping (no membership required, but it's a good deal**), the NI ones to walkni.com.

Includes many familiar (and short) paths, but also less well known routes. Longer ones include St Cuthbert's Way, Bempton Cliffs, Worcester & Birmingham canal, Ben Macdui, Fife Coastal Path, Anglesey Coast Walk, High Cup Nick, Sandstone Trail, Bradgate Park, Carding Mill Valley & Long Mynd, Gloucester & Sharpness canal, Sperrin Mountains & Beaghmore Stone Circles.

https://getoutside.ordnancesurvey.co.uk/itvs-britains-100-favourite-walks
http://www.walkni.com/walks/

Iain

* '8000 walking enthusiasts'
** £25 or so a year for the whole British mainland and Isle of Man at scale up to 1:25k