Up for offer is a small gilt antique livery/uniform button with the coat-of-arms of Dulwich College, a boarding school for the hyper rich in London, England. 

My research shows it could be a livery button for the family armigers of the Alleyn family, whose ancestor founded the school. The school stopped using the coat-of-arms in 1935 and started using a very similar one. So if it's a blazer/uniform button for the school, then it's pre-1935.

The button is in excellent condition. No backmark. It measures just over 1/2 an inch / 14 mm across.

Be sure to check out the other buttons I have listed!