Does ISO-8601 provide an abbreviation for UTC timezone and not Z as the timezone?

In several projects I've worked on, we've used

YYYY-MM-DDThh: mm: ssUTC

eg.

2017-01-01T12: 00: 00UTC

as a time format and claimed that it is (incorrectly?) a compatible subset of ISO-8601.

It is clear that the ISO-8601 includes Z

and +hhmm

as legitimate ways to offset instructions from UTC, and thereby the time zone (excluding summer savings).

It seems clear that the W3C and most other organizations adopting (subsets of) ISO-8601 prefer to use Z

(s +hhmm

).

I'm looking for someone familiar or better with a copy of ISO-8601 to make sure it is compatible with any abbreviation in the timezone in general and UTC in particular (as a synonym for zulu-time 'Z').

I believe that IANA is in charge of time zones, and ISO-8601 only uses offsets to avoid having to deal with the issue of changing time zones.

I found an older version of the standard here that doesn't mention timezone abbreviations. I would like to confirm that it is not valid in the current standard (ISO-8601: 2004?) To use UTC and requires ISO-8601 compliance.