Yeah that is the Up London Bridge Slow, which is the Up Slow before Cottage Junction.
As NSE says, you normally expect to leave on a train from Pfm 4/5 at East Croydon heading towards Norwood Junction. If you then pass Norwood Junction through Pfm 1 then you are pretty much safe although exceptions do apply via the Fast Reversible. Most Caterham trains, amongst others are booked to go that way if that helps.
One thing to look out for is that you fly over the fast lines that go towards London Bridge and then merge from the left. If you pass under the line towards Selhurst then you are on the fast line.
Edit: I see someone else got in before me.