Hobbs calling their Canterbury store an "outlet" seems very odd to me. The store is just off the high street, not exactly a hub for outlet shopping - the store itself is nothing exciting and it's mostly surrounded by generic British high street stores. There is a fairly nice Fenwick department store but that isn't cheap if your intention is bargain-hunting. I love the city itself but I think you'll be fairly disappointed if you made the trip from London especially for outlet shopping.

You didn't mention Cheshire Oaks on your list. Chester is pretty much bang on a two-hour journey from London on a fairly decent direct Virgin train, and it's quite a nice city for a day out. Cheshire Oaks is less than 10 miles from the centre of Chester so a taxi would be fairly painless.
Apparently it is "the UK's largest outlet centre, with 145 stores and was the first designer outlet village in Europe"