Cossack beat me to it!
The old Ready (RDY) message has been split into two.
Companies can now send a Ready For Improvement message (RFI), which will improve the slot to no earlier than original EOBT + Airfield taxi time (16 or 20 mins at LHR, depending on departure runway). i.e. it is assumed that pax will be boarded to make the EOBT. I'm led to believe that this is usually automatically done, unless there is some obvious reason not to (tech problems etc etc.)
ATC can send a Ready (REA) message, which can now improve the slot no earlier than the present time + taxi time, even if this will be earlier than the original EOBT.
Gonzo.