I have been looking at the following agencies:- Inkshed, Central Illustration & Eastwing. I note that both Inkshed & Eastwing do stock illustrations too.
Inkshed represent almost 30 illustrators, including Ian Pollock, Lo Cole (illustration physical illness & mental health), Leonie Lord (illustration - I love to eat sardine sandwiches), Michelle Thompson & Rose Forshall.
CIA , founded by Brian Grimwood (musicians illustration), represents over 80 illustrators, including Jeff Fisher, MH Jeeves(Illustration -woman playing violin with a saw), Adrian Johnson, Tim Marrs (Go Green illustration), Paul Wearing (black woman, white lines) & Sarah Young (figures with butterfly).
Eastwing Illustration Agency represents about 25 illustrators, including Nelly Dimitranova (wonderful linework), Rhona Garvin (swimmers), Damian Gascoigne (bike), Matthew Richardson & Ian Whadcock.