What is the box in the chancel wall?

To house the consecrated wine and bread.

It would normally have an ambry light burning in front of it.

Ambry: Middle English: A locker or recess in the wall of a church for sacramental vessels, etc.

The answer is painted on the door.

It contained consecrated bread and wine from the communion service.

Such was not infrequently kept so the parish priest could take it with him when visiting the sick during the week.

But it was kept secure against misuse.

Edit: Formal term "reserved sacrament". (url)

Many churches in England, now Anglican, are easily old enough to have been Catholic originally, and some rites more commonly considered Catholic persisted in parts of the Anglican church. (perhaps especially in the West Country)