This voluntary (number 1 of 16) was written for the large English organ of the mid 1700s, which had three manuals and could be used for echo effects. In this case the manuals would be used for Trumpet, Echo and Bass line. The organ at Ansbach was built at the same time Boyce was composing voluntaries for the organ and it has the resources to fit this voluntary.
As was typical of English voluntaries of the time, it starts with a slow prelude, before the trumpet enters. The piece is for manuals only, since English organs of the time had no pedalboard.
For details of the sample set:
https://forestpipes.de/?page_id=1793