Torquenstein:
Does it matter that the reciever is only 720p/1080i capable?
Probably not. Your TV has built in freeview which has TV1/2 in 720p and TV3 in 1080i so you are safe there. I'm not sure how you'd get 5.1 sound out of the TV to the amp though. Check the Manual to see how its done and that the onkyo has enaough inputs to take the audio - it more than likely does.
As for the PS3 you should be fine. Take an HDMI video cable out the back of it straight to the TV (that keeps your 1080p) and Audio from the PS3 to the amp would be through an optical cable for 5.1 - so make sure the Onkyo has an Optical In.
So you need to check the Amp you get has enough audio inputs to take the 5.1 from your PS3 and 5.1 from your TVand you are done.