Where do I find agency copywriter roles?

Question

I'm a copywriter working in-house in London and looking to move to a creative agency in a midweight position... but I'm struggling to scope out vacancies in agencies. There's plenty of in-house positions to apply for, but by and large I'm not seeing many opportunities within agencies, which seems strange given how many there are in this city.

I don't know if this is just the current job climate, but there's next to nothing posted online by agencies on LinkedIn, Reed, The Dots (obvs) etc
 
I'm aware that getting yourself and your portfolio out there is a great way to make connections and create opportunities, but does anyone have any tips on how to directly apply to agencies?