P&O use buses at Dover because for a variety of reasons to do with safety, security, border security etc they don't want random people walking through the vehicle loading areas where pedestrians are prohibited. If you're on a bike though you're OK apparently.
You do get a dedicated route by bike, though, that by-passes the main road through. Near the terminal, there's a gate that you get buzzed through that takes you straight to a portacabin mainly used for rebookings when things go wrong (which, irritatingly, means the queue can be quite slow-moving). You get your boarding pass here, and find yourself right by where the queues are all aligned, and get directed to a dedicated one (at the point where people get out and mill about anyway). It's actually not badly done.
(The cycle boarding at Harwich, which involves going with the rest of the vehicular traffic over the concrete ramps and twirly bits over the railway line, is far scarier!)