It depends on how the care home was bought / transferred. If the company was bought outright by the new owner you are in the safe zone.
If only the assets were bought you have a problem. This is where employment law and immigration law collide. Unfortunately immigration law overrides employment law in most cases.
You will have had your employment rights transferred through TUPE see
http://www.direct.gov.uk/en/Employment/ ... G_10026691 for basic information.
However TUPE does not confer the transfer of immigration entry conditions. The WP will only be valid if the whole company as an entity was bought. WP is issued to a business entity therefore if you are no longer employed by that same entity you do not have a valid WP.
I have seen cases where the employees are not even aware that a new owner is in place, operating under a different business or legal entity. They only find out when there are noticeable changes like the insurance certificate for public liability insurance states a different company name. This is extreme as there is a legal requirement within TUPE to inform the employees. At the low end of the employment spectrum there are many employers that have a total disregard for employee rights.
