small / cheap I use hermes, 2-3 quid for a small parcel
small / mot cheap I use post office, 4 quid second class signed for
Medium it gets tricky. I use parcel2go and tend to send with hermes for cheap stuff or UPS / parcelforce for more valuable stuff (wheels, frames)
full bikes, I have used senditnow, its post office really but a bit cheaper. send a full bike box for under £20, but i think they have just priced themselves up with everyone else so more like 30-40 now for a big box.