A few points :
1. If LCY is fogged in one morning, Stansted is closer and more convenient by road than Stansted. I presume they could use the exec terminal.
2. Shannon stop westbound works only if you do US customs preclearance, when there can be a notable saving in time. Interesting to see how the resulting "pressure" on the US immigration staff stationed there for an on-time departure is handled, but for those routinely queueing at JFK for 1-2 hours on arrival it could be even more of a timesaver then the eastbound nonstop.
3. The world has not stopped economically, despite what the downmarket news media may care to write about. There are still plenty of pax in premium class on BA/Virgin/AA/DL/CO from London to New York.
4. Having BA as an established operator on the city pair doing this gives all sorts of advantages over the recent independent premium service operators on the route.
5. BA already has a large and established client base on the route, with plenty of Golds and Silvers, and likely has already struck volume-based corporate deals.
6. Not a lot of demand for early morning departures from LCY as target clients do not live or have hotels near there. Aim for those who go to Canary Wharf offices first. A lunchtime and early evening departure out of LCY will work best. In the opposite direction need overnight flights as early as possible (into LCY just after it opens) and as late as possible out of JFK. A BA exec bus/limo from LCY to/from Canary Wharf and Bank would be a real benefit.
7. If there really is an A318 on the ground at LCY for most of the day seems OK to schedule it on a trip to Europe.
8. Interested to see what they do with the aircraft at weekends when LCY is closed and when there will be little demand westbound on either Saturday or Sunday.