this requires the editing of a few files. i suggest you read the tutorial for adding a new unit to the game here:

https://forums.totalwar.org/vb/showthread.php?t=75706



basically, you need to edit export_desrc_unit.txt in your ..\data folder to include england as owners of the Zweihander unit. then, you need to edit battle_model.modeldb in your ..\data\unit_models folder to include england as a country who has access to the Zweihander unit. next, you need to place the appropriate #zweihander.tga file in folder ..\data\ui\units\england. finally, you need to edit export_descr_buildings.txt in the ..\data folder to add england to all the places Zweihander units are built, otherwise you won't be able to build them.

the ..\ stands for your M2TW root directory (the place you have it installed). this is all needed so that you have icons and everything and the look is the same for England as it would be for the HRE. the only issue here is that your units will still wear the HRE meshes, but they will hold your flags. i'm not skilled with re-meshing, so maybe Alpaca can give you some guidance there.

if this all seems to be a little too much and you don't mind having generic unit cards in place of the actual zweihander unit cards, then just edit the export_descr_unit.txt and the export_desrc_buildings.txt files and that should be enough.