The British monarch needs permission from the Lord Mayor of London to enter the City of London.
Numerous sources which confirm this:
This is purely ceremonial, the British monarch can comes and go as (s)he pleases in the City of London.
Sources Mick's provide to prove his case:
1. Wikipedia cites an unpublished art student's paper, which in-turn cites an unpublished English Heritage report - Nothing then.
2. The City's website, which confirms the monarch asks permission to enter the city - Thanks for that one.
3. A book which mentions the Protector (not a British monarch) having to perform the ceremony too - This doesn't help Mick's case.
3. A book (snippet view) which mentions the ceremonial - So what?
A bunch of Google results will also tell you that aliens abduct people and implant things in their bodies. Perhaps you could point to some sources that say this is anything more than a tradition (which is actually what most of the Google results say)?