Sky remote. It can be programmed to control the Onkyo for power on/off and volume which is enough if all the source is mainly is the Sky box anyway.
Harmony remotes have the volume market sewn up above the basic 4-way/6-way coded remotes. Above Harmony then there's the fully customisable remotes, but I suspect that £600 for hardware + programming cost is somewhat OTT for the system as described.