There's always a reason to delay - as you say when your eldest in is year 6, your youngest is then in year 3 so would the same reasoning apply and then you'd want them to complete year 6?
If you're intending a permanent move, my opinion is that the sooner the better, to allow them to settle into the new education system as soon as possible. Also remember that due to the different semester breaks etc, the year they are in over here may not be the best year for them to join over there - it's not necessarily seamless and it's quite normal to change year group to allow for settling into new system.
I'd also try and remember that it's easier, again in my opinion, to move a year 3 than a year 6 as one would hope that they're not so entrenched in the UK and more adaptable to the change.
Good luck - as ever, decisions with kids and migration are never easy.