I'm pretty certain that there is scope for extra capacity to London. Remember that we are around a third down on its highest level.
I won't perdonally be making use of the service as I have three reasons for flying to London - visit my sister (Heathrow or Gatwick), visit my company's Thames Valley office (Hearhrow) or connect (Heathrow). But there will be plenty of people with destinations in the city or around Canary Wharf who will find this suits them.
Where they may find they can get some joy is is they can have a departure to London around 6:00-6:30am. The current offers from EI and FR don't allow you to fly over in the morning and get into an office at a reasonable hour.
I think what surprised people with this one is that nobody had seen it coming. As I understand it, it doesn't qualify for route support, so it needs to perform from day 1. As such, it's a bit braver than a lot of the routes proposed.