Have you tried turning off any privacy/cookie blocking features? I always found those to affect discount codes on the EE Store site. Possibly try another browser also.
Otherwise you would likely need to raise this internally. There may be links on your company intranet or someone you can contact around discount schemes and incentives.
I will also add it may not be appropriate to post about the details of internal BT Group discount schemes or to reference the terms used for any internal employee identifiers on a public forum such as this. The latter could potentially cause a risk to the business via means such as social engineering.
