Does a line up and wait instruction clear me to backtrack the runway? I had an interesting occurrence at the start of a recent flying lesson, at an aerodrome with ATC service. We were holding at A and completed our power checks. We hadn't requested nor been offered an intersection departure, and that is not a usual procedure at this aerodrome.

"G-AB ready for departure"
"G-AB via A line up and wait runway 23"
"Via A, line up and wait, runway 23, G-AB"

Following this, I entered the runway and turned to backtrack to the threshold. At this point my instructor said I should have turned the other way to line up at the intersection, because they didn't clear us to backtrack. Was he right to complain, or was I correct to use the full runway length?

Notes

A is only about 20 m from the runway threshold, and this runway is huge, so it makes little practical difference in this case.

At this aerodrome, they usually give LUAW instructions as "via [point], enter, backtrack, line up and wait runway 23". I checked CAP 413, and it doesn't mention "enter, backtrack, line up and wait" as phraseology, so I'd expect a clearance to LUAW entitles you to the whole runway unless you've already agreed an intersection departure. However, it's CAP 413 section 4.24 that introduces the "via A" phrase, and it has this to say:


  When line-up will take place at a position other than for a full-length runway departure the intermediate 'Holding Point' designator shall be included in the line-up instruction. Controllers may include the runway 'Holding Point' designator in any other line-up instruction when considered necessary.


It gives an example, and the only other example with "via" is in the context of an intersection departure. As I said, at this aerodrome they usually include the "via"; maybe they just always consider it necessary. Is not saying "backtrack" here significant, even though it's not a standard part of a LUAW instruction?
Your instructor was correct. According to the CAA Radiotelephony Manual[pdf], since the clearance you received included "via A" your clearance was not for a full length takeoff.

From the Radiotelephony Manual, chapter 4: