The breakfast options caused a bit of confusion for me when planning to catch the sleeper last year, but I now understand how it all works for standard class passengers. As mentioned previously in this thread, you can pay £4 for a breakfast to be delivered with your wake-up call, but note that standard class passengers only have the choice of the continental breakfast, not the hot option available to first class travellers. The man in seat 61 suggests that the £4 standard class breakfast supplement can get you a hot breakfast, but it cannot.
Having said that though, I thought the continental was pretty good for £4. I can't remember exactly what it contained, but I do remember a hot drink, juice, yogurt, fruit salad (of decent quality), and croissant.
Also, something I haven't seen a mention of anywhere is that if you forego the £4 continental breakfast option, the lounge car actually has a hot breakfast menu in the morning. I can't remember what was on it other than bacon rolls though, which the consensus here suggests aren't very nice though I never tried one.