In the UK, there's St Marks Hospital,, which is within the Northwick Park Hospital complex, Harrow, near Wembley Stadium, London.
St Marks Hospital is regarded as a centre of excellence and is the only Hospital wthin Europe that specialises solely in intestinal and colorectal ailments, complications and Surgery.
In fact, the J pouch was pioneered at St Marks Hospital.
St Marks is a NHS Hospital, so referal from a GP (physician) or Consultant is normally required, however, when I had my J pouch surgery at St Marks, I did meet a number of patients who claimed they were receiving treatment privately via medical insurance