Whitbread said he was not being replaced.
Kerr and PPS directors John Mills and Nick Jones will each head one of three teams covering London, the South and South-East.
The firm has also appointed Will Hansom as an account director in its Manchester office. He joins from Chime Communications-owned TTA Group, where he worked for clients such as housebuilder Crest Nicholson. Hansom was previously property comms manager for Tesco Stores.
Also joining the Manchester office as an account executive is Danny Langley, who previously worked at Stockport Council in the post of scrutiny officer.
Labour activist Alex Barnicott and Tory campaigner Rachel Tooby have been hired to bolster the London office as account executives.
The appointments follow the arrival in February of Mark Cummings, from the Association of Scottish Colleges, to head the Edinburgh office. He replaced Colin Pyle, now RBS media relations manager.
MD Stephen Byfield said the consultancy currently numbers 42 staff, including 25 in London. Fee income for the year to end of May is expected to be around £3.5m.