It will not be a BT Engineer, it will be an Openreach Field Technician.
BT Retail do not have any engineers who work on the external network.
Unless Openreach are working on your line, or doing major work on the cabinet, your service should not be affected.
You know when I change providers, will openreach go to the cabinet? to change activations?
Yes, they would normally visit the cabinet to change over the routing unless you were moving to a provider who use the BT Wholesale network, and there is no waiting list for ports.