In that sense it's a crying shame that BA don't / won't act as what a flag carrier should be, as they are the operator that could cope with a need to change between big and medium aircraft as necessary.
Instead everyone loses; something looks like a good medium aircraft route so someone different tries it out with a big aircraft, it doesn't work, it gets dropped and may or may not be picked up by the same person / someone different, there may or may not be some rinsing and repeating as necessary, and finally the route is dropped completely (perhaps because the passengers have lost track of all the changes and got p'd off).
The only good thing for flybe is that a partnership with someone, or perhaps the people, in the business of flying small aircraft means they might not be so liable to lose out to the same thing in the other direction; some of the routes they'll be dropping might have viable futures as Eastern's type of route.
I'm sure there may be CS100 opportunities in the UK, but I'm not sure who would be the people to try and take advantage of them. It's not flyBe as their finances don't allow it and their shareholders would never let them take a risk of similar appearances to that they took before with the E195s.
The only people with the finances to try it would be easyJet, but their shareholders also wouldn't be happy with it and it would be an awful fit with the business model that they're pursuing anyway. I wonder if the UK's CS100 opportunities might be taken individually by flag carriers other than BA?