I can only speak from experience but Ive found my black tribal faded and partically removed entirely very fast and very well I had around 4-5 sessions for a colour cover up and would estimate 10 would have obliterated the entire tattoo.
Luckily for us black is the easiest to remove

Scarring depends on many factors including how well the tattoo was put in to begin with, problems may not always be visable on the surface and scarring can be kept to a minimum by good aftercare and massage (when healed)
I dont know of any studio lasers in london (tho Im sure they will be in abundance) woody in High Wycombe is a self oppointed expert and yell.co.uk brings up multiple skin laser hospitals and clinics...