I've run into a few that were on the
Grauniad jobs site (effectively the online version of the old Tuesday Educational Jobs supplement) rather than THES or jobs.ac.uk, but those three sites will cover 99.9% of UK academic jobs.
For JRFs it's probably useful to still keep a weather eye on the
Gazette and the
Reporter as well as the
colleges page at Cambridge's HR department as they do tend to have the adverts before the national media.