So sorry you have been going through all this.

I wouldn't really worry to much about core specific strengthening until you get to the root of your problem or you might just light it up even further.

From personal experience of being stuck in a hospital bed for 6 weeks after my accident I can attest that just by walking and targeted isometrics that you will quickly gain those muscles back even though they feel so weak right now. Doing the isometrics is important because muscles actually have memory so doing them will help you anytime pain keeps you down especially for the large muscles of the leg/butt/thigh. For your abdomen you can do something as simple as tightening and pulling in your belly button toward your spine, hold it for a second and release, then repeat. Doing that either sitting or lying down will help a great deal.

You might find that B12 along with Vit. D3 will improve your energy levels. Another thing that would help is massaging your (in places that you can) to rid your muscles from lactic acid buildup which can happen if you are mostly immobile - that lactic acid can cause fatigue, muscle cramping and weakness. A rolling pin is an easy tool to use for this purpose, simply roll your muscles and if you find painful trigger areas try to work them out by light pressure and gentle massage.

I was curious if there was a reason your GP put you on such a strong narcotic for your slipped disc to begin with? I ask because at least here in the US that wouldn't necessarily be the first choice for that type of pain and also for someone who doesn't take pain meds often.

I hope you feel better soon!