Fuel costs a shedload on the Channel Islands, so it is cheaper to take round trip fuel whenever going there. The minor additional fuel burn is not much of an issue because the sector is so short, and the price penalty so bad, but you wouldn't take fuel for more than 1 return trip apart from fairly unusual circumstances.
The fuel burn would depend on the type, and several different types could conceivably serve that route at present (ATR, RJ-100, B737).
It wouldn't generally be the same aircraft going backwards and forwards all day, but several different aircraft handling the different rotations, plus other routes before / after.
CPB