Are you Safeguarded? Non-Safeguarded staff are not entitled to Priv rate travel on London Underground only tickets, except for certain staff employed by LNER (East Coast), GTR and Eurostar, as well as children of active LU/TfL staff. If you however are buying a single/return ticket for a National Rail journey that requires you to cross London via the Underground (for instance a journey from Surbiton to Stevenage), then you can use your 75% discount for this.
If you are Safeguarded, then the best option for ad-hoc trips is to have the discount added to your Oyster card. This is done by completion and authorisation of a form available from Rail Staff Travel (
https://www.raildeliverygroup.com/rst/travel-on-tfl.html), with the discount added to your Oyster card by LUL staff on presentation of this authority.