This is covered by the TUPE legislation, some basis info here...
http://www.out-law.com/page-448
"As the new employer is required to take on the employees on their existing terms and conditions of employment, it is prohibited from making any changes to the terms and conditions of employment of the transferred employees if the sole or principal reason for the variation is connected to the transfer (unless there is an ETO (Economic Techinical or Oganisational) reason for the change, usually requiring a change in number of the workforce)".
Some more info from ACAS - they may be able to advise you fruther...
http://www.acas.org.uk/index.aspx?articleid=16 55