I don't know what you mean by a bad winter, but neither jasmine nor camellias are very cold tolerant. Most camellias are rated hardy to USDA zone 7 which means to about -15C or 0 F. The confederate jasmine (trachelospermum) is only rated hardy to USDA zone 8 which means about -12. Ã‚Â°C (10 Ã‚Â°F).
So if your winter was any colder than that, they were just cold killed. And even above that, it may still have been too cold for them. Even though they are rated to zone 8, they usually say to bring the jasmine indoors when it gets below 40 d. F.
You were right about the ericaceous compost (had to look that one up, it's more a UK term), both these plants like acidic soil. The jasmine likes slightly acid to neutral, the camellia likes quite acid. But in looking it up, I found that sometimes what is labelled ericaceous compost turns out to be 100% peat. Peat is acidifying, but it is problematic if you put plants in 100% peat (did you mix it in with the soil or just plant into it?). Peat tends to hold too much water, both these plants like to be well drained. But if the peat does dry out completely, it sucks water away from the plants and then is difficult to wet again and water runs off it without soaking in.
If your plants are dried up with brown brittle leaves it may be too late to save them. I would cut all the dead stuff off and take good care of the roots, see if your plants will come back from the roots.