The big one in London is Bracken. Site in Elephant & Castle and a small site off the Marylebone road near Euston. Bit pikey looking. Sort of place that I would avoid. Seen them laugh at customers that aren't too technical (i.e. like me) .. even though the chap had two spanking blinged up bikes from them. One for him and one for the girlfriend. Sales guys taking the piss after they walked out. I would recommend staying away with a barge pole.
There is another one called InMoto in Croydon. Don't know much about them.
Then there is the KTM centre in hemel hempstead. Seems like the sort of shop I like. Two owner company, buy/deal direct with owners rather than disgruntled employees. Didn't seem like the sort that would take the piss out of people that give them business like Bracken.
Same thing at Tricountry motorcycles in Bracknel. Seem like down to earth blokes.
Then there is P&H motorcycles in Crawley. The KTM stand is manned by the bloke that used to own his own KTM shop. Sussex superbikes or something like that. So he knows his stuff, but not sure if attitude is different now part of a big shop.
THere's your KTM choice in the SE ... I wonder why I know so much