You can spot these a long way off, one reason I filter on the outside as much as possible. They'll get surprised by an overtaking motorbike and crush you against the kerb in an instant. A helmet camera makes for a high-quality police report to discourage them from this habit with 6 points and a fine.
I was tempted to filter on the outside, but the other lane was relative fast moving - I didn't feel safe doing that. The crawling left lane was traffic heading for the motorway I guess.