The current situation is that the airport taxi concession holder provides a service in that situation. This is explained on the SEN website.
In the case of the late RNS flight there should be sufficient pax to benefit from their £20 charge for a minimum of 5 people to London Liverpool Street. If fewer than 5 people need the service then the normal £100 cost is shared by those needing to travel. Not ideal but at least it means that nobody is stranded without any means of getting to London.