Hi superpilot,
BA's renowned "super shuttle" services are basically services from Heathrow to Glasgow, Edinburgh, Belfast Aldergrove and Manchester. They are high-density flights, operated by BA's 757/Airbus fleet. They are called "shuttle" flights because they are designed for commuters, they are quick, frequent and (supposedly) good at running-to-schedule. Many businessmen use them to go to and from meetings in the big cities of the UK.
Shuttle's have the callsign "Shuttle" followed by a number and a letter, eg. Shuttle 2 Romeo would be Heathrow to Manchester, Shuttle 3 Tango would be Manchester to Heathrow, etc. Not all BA Domestic servies from LHR have the "shuttle" name however, Aberdeen and Newcastle operate as normal BA servies with normal BA callsigns.
I hope this answers your query.
All the best
GBPEC