Jack,
I don’t trade fx but for what it’s worth here are my thoughts.
Looks like the Yen has been strengthening against the majors of late, either through unwinding of carry-trades or anticipating such.
I wouldn’t argue with your short UK/JPY but what about, simultaneously, short US/UK? You’ll have long yen (short pound) exposure on the UK/JPY but you’ll also benefit from what I expect will be strengthening cable. My thinking here is that the majority of carry-trades will have a dollar bias so when the dollar is sold, UK/US will strengthen. Will this then be a pound-neutral/short US/JPY position? Maybe I need to get out more.
Trust the Japanese? Are you with Alec Guinness or Jack Hawkins (“Kill him, kill him”)? Jack, any day.
Good luck.
Grant.