Using UK iPhone in Japan

OK, I'm heading back to the UK for a holiday this summer. I'm debating buying a brand new unlocked iPhone 5 (can get the VAT knocked off, so cheaper).

Question is... will it accept my Japanese YMobile SIM when I get back here? I know Japanese phones are usually locked to networks, but are SIMS locked to particular phones?

I'm currently using a Kyocera DIGNO T on YMobile.

Any advice appreciated! Thanks

Y Mobile sells iPhone, so I don't think they accepts oversea made iPhone with their sim cards.
MVNO companies include b-mobile and biglobe possibily accept oversea made iPhone.

I believe that you have Ymobile phone, because you are a resident of Japan.
if so, you can't get it with no sales tax.
if so, your Ymobile sim will work on your iphone, because they sell sim card only.
https://online-store.ymobile.jp/regi/?oid=SIM