Ive had a look at the Canix races, they look great. I'll have a look at theDorset Doddlers Seela. There seems to be more opportunities than I thought, it's a shame that you can't run with dogs on all races but I guess falling over leashes could be a issue !!
I use a camelback style of rehydration system, the major downside I find (IMHO) is that for the longer duration runs the water heats up due to the conduction of heat from your back, this is made worse on hot days. A lot of the serious ultra guys dont like them because of the time spent re-filling them at check points in comparison to the bottles, and its difficult to monitor your hydration levels because of not realy being able to see it.
Other than that they are great bits of kit, the load feels well balanced on your back. Top tip though... get all of the air out of the bladder before closing it, the sloshing noise drives me nuts!! is this just me?