I think you may find it does not like its position in your garden. They need warm conditions which is unlikely in a north easterly position, they do not like wind. They also need to be grown in well drained/light soil. One suggestion is to grow them near a warm wall another is to grow in a container and move into a cold greenhouse in winter.

Choisya grows really well here in London, even in exposed situations. As the leaves have started to go whitish, that usually indicates sudden temperature change - its possible the plant you bought had been kept under shelter, and if you took it straight home and planted it outside, it's got transition shock. Bring it back under shelter (not necessarily indoors, if you have a greenhouse that will do, otherwise somewhere sheltered outside and bring it in at night till it recovers), keep it watered, give it a feed and hopefully it will recover, ready to be planted out AFTER you've hardened it off properly in a few weeks/

Around here, whitish leaves mean that the plant was grown in a plexiglas greenhouse, and never developed any resistance to ultraviolet. Plants can sunburn just like people, even under minimal sunlight!

That is a very good point TugB