Free, legal, musically accurate, and perfectly usable for most choirs. Weaknesses: Lacks the polish and rehearsal aids of a commercial edition; page layout can be cramped.
Download the CPDL version for an initial read-through or a low-budget performance. If your choir loves the piece, consider buying the Novello edition for long-term use. For a one-off church service, CPDL is more than sufficient. To find it directly: Go to cpdl.org → Search "Lux Aeterna Elgar" → Look for an edition by Edward Elgar (arranger/composer) → Check the edition date (preferably pre-1923 for US safety) → Download PDF.