Mine was 10 when we moved abroad, and he didn't want to board then, so we bought him with us. He could have stayed at prep and boarded.
He turned 16 last year. We had a discussion about what was available at sixth form here, and what was available at home. Where he is going (Symonds in Winchester) is NOT boarding school. It is a state sixth form college that happens to have two boarding houses and about 80 boarders. I went 30 years ago this September and had the best two years ever. I am sending ds because he will be going in two years time anyway for Uni, so this is a chance for him to live away from home, but still very much supported, and learn to cope with doing his own washing, managing his own money, and generally being back in the UK. I don't want him to go to uni and fall flat on his face because he can't manage!
I didn't think he'd want to go, but having heard me talk about it, and having attended an open evening and then a tour and interview, he is thoroughly looking forward to it, although is understandably a bit nervous.
They change greatly between 12 and 16 believe me, and are more willing to accept that boarding is an option when older. You will need to look long and hard at the results the school you are going to use gets at IB, and the subjects they offer. IB is not for everyone.
I would also add that you only have to pay boarding fees for state boarding and not the tuition you will be paying for an international school. It will save my husbands employer a shed load per annum if they pay for him to go to UK, and if they don't, the sum isn't so large that we can't afford it.
Boarding for sixth form isn't an option you should dismiss; it is worth doing some research so you know what is out there, and some forward planning. I didn't know when we moved that we would still be here for sixth form. We are due to move part way through sixth form, although dh could get another job here, or anywhere in UK, so ds has to be in one place.