It's 'protest activity' rather than someone buying ad-space legitimately so it bypasses TfL policy, and the Advertising Standards Agency code/complaints procedure.
As for the symbol, in the UK the law focuses on intent. Only an issue if used as 'hate-speech' to incite/harass.
Comments
https://www.instagram.com/everyonehateselon_/
https://www.gofundme.com/f/people-vs-elon-campaign?attribution_id=sl:35e0bce7-3308-4041-99c8-d8ab98f4c8e4&utm_campaign=pd_ss_icons&utm_content=amp13_c-amp14_t1&utm_medium=customer&utm_source=copy_link
As for the symbol, in the UK the law focuses on intent. Only an issue if used as 'hate-speech' to incite/harass.
1) The ad agency getting it approved by the ASA (not compulsory, but 'good practice').
2) TfL etc accepting it (easier if #1 has happened).
3) ASA ruling against it or TfL changing its mind if/when complaints are made: 'taking it down'.