Update: So today I went to Royal Mile with the aforementioned Raymond. It was a very good, if exhausting day. Highlights included African Dwarf Kingfisher, Spotted Greenbul, Ituri Batis, Fire-crested Alethe, Cabanis's Bunting, Zebra Waxbill and Grey-headed Oliveback. And a Chimp. A few other targets were heard but not seen.
Raymond confirmed that you're supposed to have a guide at Royal Mile. I paid 90,000 shillings for half a day entry. We visited a few other sites in the area, including the farmland areas and 'the pond'. Raymond was excellent and worked very hard to find things. Note that he doesn't have a speaker or laser pointer, so you might want to bring those along if you have them.
There's very little infrastructure at Royal Mile, just a gate with a guy manning it and a guy who cycles around collecting the fee.
One thing to note is that the road from Masindi to the Budongo area is a bit of a laugh at the moment as it's basically being converted into a new road, which will be very fast once it's done. This makes it a bit tricky and dusty in places, although it's generally pretty obvious where to go.