I live in LB. Traffic starts getting thick, between DLR and LB, around 4pm. And by "thick", I mean a drive that typically takes 17-20 minutes can take almost an hour. The good news is, boarding in LB is quick and painless. Still, if it were me, I'd be in that taxi by the time the clock turned 4pm and headed to the airport. And I'd be praying that the driver knows how to use surface streets, just in case.
The only downside is- if you get to the LB airport a shade too early, it is the world's most boring airport, lol!