The oldest existing Seventh Day Baptist church is the Mill Yard Church in London. Chamberlen may have been the pastor from its beginning in 1651 to the time of his death; but whether he or John James started the church is uncertain. He was the leader of the Whitechapel Congregation (the precursor of Mill Yard) by 1653. 
One of Dr. Chamberlen's strongest stands on the Sabbath was his participation in a debate in 1659 held in the Stone Chapel beside Saint Paul's Cathedral in London. Jeremiah Ives spoke against the Saturday Sabbath, while Dr. Chamberlain, Thomas Tillam, and Matthew Coppinger spoke for the Sabbath. Woodham Mortimer, Essex, England, is Chamberlen's burial place. The inscription on his tomb reads as follows:
The said Peter Chamberlen toock ye degree of Doctor in Physick, in fever all Universities born att home and abroad and lived such above three score years being physician in ordinary to three Kings and Queens of England. viz. King James & Queen Anne; King Charles ye first & Queen Mary; King Charles ye second & Queen Katherine; & also to some forraine Princes; having travelled most of partes of Europe and speaking most of the languages.
As for his religion he was a Christian keeping ye Commandments of God & faith of Jesus. being baptized about ye year 1648, & keeping ye 7th day for ye saboth above 32 years.
To tell his Learning and his Life to Men: Enough is said by here lyes Chamberlen. 
1. Sanford, A Choosing People: The History of Seventh Day Baptists, Broadman, 1992, p. 62