The problem with LCY is not demand but profitability.
By the time you factor in high charges there and the fact it doesn't qualify for route incentives in Cork, you are facing a pretty hefty set of costs.
The destinations there can pretty much be divided into three
- Routes where there is heavy business demand that will tolerate high costs (this is most of them)
- Routes where it's the only London route from the source airport (e.g. Antwerp, Exeter)
- Holiday routes that are weekend-only
There are a couple of routes that don't fit that pattern (Billund, Vilnius), but I'm not privy to their economics and how they manage to work.
One thing you can say though is that Cork doesn't really fit into any of the three buckets above.
The major components of business traffic in Cork are Pharma, IT and Agri-business. With the exception of a small bit of IT work, central London and the Docklands area is not the destination for those. We have no significant Financial Services or Legal Services base.
To be honest, I'm not sure to what extent an A220 might work on UK routes. It might be the right aircraft for Manchester or Birmingham, but the ATRs and Dash 8s are probably right for the other routes. And while there are a few places where freeing up ATR capacity might be nice (extra frequency to BRS or EDI, or restarting SOU, maybe looking at somewhere in western France like Nantes as a year-round option), it's more likely that introducing an A220 and Stobart losing the highly profitable MAN and BHX would undermine the overall base and may lead to the loss of the ATRs altogether.
The only other use I could see for an A220 for UK routes would be to operate twice daily to Gatwick. It would be a feed for BA's mini-hub and Gatwick generates decent demand on it's own. But there are a few problems with that. Gatwick's set up encourages increasingly large aircraft and they won't be delighted to welcome an A220. Slots aren't exactly easy to find at Gatwick. Ryanair would probably react and make the route loss-making. BA's presence at Gatwick has fallen from 40% of slots to 20% of slots, so they don't really look all that committed there.