Two scenarios really.
Lets for arguments sake say its been built for the 1997 changes came in force and you can prove it.
A/ You don't have to do any thing
If it was built after 1997 or it cant be determined when precisely it was built, it will need a IVA by VOSA where the age will be determined from the CHASSIS number, unlike before where its number plate was derived from the body.
Personally I think you will enter a world of **** if you have to go through a IVA, if the chassis has been altered in any way they will need documented build details and the whole vehicle will be inspected as if it was a brand new Land Rover product, and lets be realistic here its a Land Rover we are talking about, it will fail.
A friend of mine did a 100 inch Discovery pick up with a Defender body on it cost him well over £1500 just to get it done through VOSA, he had photos of nearly every weld and modification done to the chassis it was inspected as a new Land Rover and was issued a new age related plate, not sure if they do a Q plate any more.
You got any pics of this Hybrid, and details of how much?