From Jersey back to Tokyo is not an easy travel. From Jersey to London, most likely you may fly with BA, as the long-haul flight is BA for more reasonable priced ticket, then you should be firstly touching down at Gatwick. Then 70 minutes of journey to Heathrow by coach follows. As typical flight schedule from Heathrow to Tokyo is around noon, considering this extra 70 minutes journey, you may have to catch the first flight back to London, but early morning flight has a risk of cancel or delay due to fog thanks to geographic isolation of isle of Jersey by relatively warm sea. So you had better leave there in the prior day and have a sleep somewhere in London.
All we know, central of London is more expensive for nightly rate, and less quality with typical airport hotels; except Sofitel London Heathrow. It stands at the prime location, just 3 minutes away from Terminal 5 (no need to walk through a long tubular for "just SIX" minutes form Terminal 4:-), and very sophisticated appearance and services, facilities, and restaurants and bars which are uncomparable with typical airport hotel.
Among them, my favourite is wider range of buffet at Vivre. While I recommend you to try their "Miso soup", which is nice to try before back to Japan, their omelette is as above. Not that bad as similar in London, many folded one, with your a pick of salt and pepper needed, yes, you are yet in London :-)