Hi @johnbooth262 and welcome.
Have you had BT service previously? Sometimes the result of the credit check doesn't meet our criteria. So we ask you to pay a deposit. It's normal to ask for this and doesn't affect your credit profile in any way. Six months after signing up, we'll give you the deposit back.
I think you will find that its the case for all new providers, they need some security in case people do not pay their bill, as they have been caught out in the past, by people running up huge bills, then disappearing.
A mortgage is a different thing, as the house is offered as security