A difficult question! There are several factors to take into consideration
If you buy in the UK you can find a time that suits you. Shop around on the Internet before you make your appointment . Many top boot fitters will spend a very long time with you and they have real expertise. Some of them will offer you a guarantee that allows you to come back if they are uncomfortable. The downside is that they are not in resort with you when the problem strikes
If you buy in resort you will only be offered the makes that the local shops have and all the time you spend fitting your boots is coming out of your precious skiing time. If you are booking your own flight try to arrive early so you can make time on day one for your appointment. The advantage of this is that if the boot is uncomfortable you can call into the shop after skiing everyday and show the fitter exactly where it is sore.
At the beginning of the season some shops will hire brand new boots. If they are comfortable ask if you can buy them instead of paying the hire fee. That way the shop is guaranteed either a sale or a hire and you try before you buy. Win win if you are lucky enough to get a shop with the right boot in stock to offer you this opportunity. Even boots that have been worn a few times may be worth buying if they can spare them from their stock
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