All medical schools welcome the offer of a donation. However, certain medical conditions may lead to the offer being declined. Medical schools can give you more information about these conditions and any other reasons why a body donation may be declined.
Post-mortem examination (sometimes referred to as an ‘autopsy’) is an important reason why a medical school might decline the offer of a body donation. We recommend that potential donors and their families are prepared to consider alternative arrangements in these circumstances, which can arise unexpectedly. Depending on the circumstances of a person’s death, a Coroner might require, by law, that a post-mortem examination takes place.
It is important to note that medical schools might not be able to accept donated bodies during holiday periods, such as Christmas.
from
https://www.hta.gov.uk/faqs/body-donation-faqs