– Nkoda (subscription) offers the Boosey & Hawkes study score in high-resolution PDF for streaming/download (while subscribed).
Digital editions and study scores are available through various academic and sheet music platforms: Official Publisher Information : Reference materials and program notes can be found via Harmonia Mundi Academic Repositories britten les illuminations pdf
Here is a feature specification for a tailored specifically for musicians learning this piece. – Nkoda (subscription) offers the Boosey & Hawkes