I’ve enjoyed a few lately.
Corrieyairack Pass – start in Fort Augustus, down the Great Glen Way to Gairlochy then on road to Roy Bridge. Up Glen Roy (which is stunning) then up and over the pass back to the start. About 100km.
The Fife Coastal Path from North Queensferry to Leuchars is good, about 120km then a train back to the start.
Obviously, any trail centre will have good stuff.
Borrowdale Bash in the Lakes is nice XC.
As thegeneralist says, the West Highland Way is good, apart from the bit by the north east of Loch Lomond.
This loop of Glen Garr is nice-
https://www.trailforks.com/route/little-glen-shee-and-glen-garr-loop/
Long XC loops are also a nice way to explore the Yorkshire Dales and North York Moors which otherwise lack gnar. You can also do some nice loops in the White Peak.