where do you dispose of a body ....in a remote place at a quiet time.( so not in Central London)
There are ways - clearly, as it was done.
The most obvious way is to lure her into a property, kill her inside, then hide the body at that property. Lever up the floorboards of my living room and a few feet underneath is the dirt the house was built on. Some period houses even have a handy access hatch to enable entry to this space, for plumbers, pest catchers and so on. If she were taken into such a house, or even an unoccupied ground-floor flat, that's where you hide her. She goes in unnoticed, never leaves and would be undiscovered until the house is demolished.
Another way is to lure her into a rented garage, Superhire warehouse to which you nicked or copied the key, or other utility building. The killer drives in with her, closes the doors, kills her, and does a
Sexy Beast disposal where he takes up and then replaces the floor tiles. The garage / lockup is then put back into normal use.
As it's a garage, he could instead drive her out again, perhaps in a different car. Then he just has to find a place to dispose of the body. Into water would be simplest, provided he can find the right water.
I don't really buy the canal story for a number of reasons, but one is the faff. Why would you hump a heavy suitcase with a body inside on foot in order to drop it into a canal that's quite shallow and has water traffic? He had her in a building somewhere if he managed to load her into a packing case. Given this and that he used a car, wouldn't it be a lot simpler to drive her to the nearest unobserved deep water with a handy road bridge over the middle?
I used to live and work around Birmingham and the West Midlands and Staffordshire. On my wanderings there as a sales rep, I noticed
Blithfield Reservoir. It's unobserved deep water with a handy road bridge over the middle, nicely secluded and half a mile or so as the crow flies from any building. It's also nearly 50 feet deep. It's untreated water, meaning if you drop bodies into it, nobody's going to start complaining the tap water tastes funny lately. I mention that one only because it's quite handy for Cannan's mother's house, for whatever that's worth. There must be many, many similar places around the country.
So how you hide a body in central London is, you get into a building and hide her under a floor; or you kill her inside the building, leave inconspicuously with her in the boot, and thus don't hide her in central London at all.