There are plenty of RJs around - one of the more plentiful types available at present. For example, there are the last of the formr Northwest fleet still available.
But with the new fleet of Embraers on the way by the end of the year it is an opportunity to avoid the costs associated with bringing another aircraft in, bringing the cabin up to BA standards, etc. BA have a contract with Titan for a standby aircraft as required, if they don't want to take that up in the medium term they are using this as an excuse for cost-cutting. A competent airline financial planning team (one thing that BA do possess) will have a provision for costs for exceptional events. Unfortunately it seems that budget has been raided to support the corporate bottom line. Whether this would have happened if the LCY operation had been part of Mainline instead of Cityflyer is a topic for discussion.
If only the competition on LCY-Edinburgh were still Scot Airways with their excellent cabin service they would have lost more. Unfortunately that has gone, in favour of the Air France (alias Cityjet) corporate approach which is not nearly as good.