I got a visa with my mother when I was 17 years old. On it was stated "Not valid unless accompanied by my mother".
Can I travel to the UK alone now that I am 18?
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The controlling technical reference for this situation is spelled out in Paragraph 321 of the rules...
A person seeking leave to enter the United Kingdom who holds an entry clearance which was duly issued to him and is still current may be refused leave to enter only where the Immigration Officer is satisfied that:
... not applicable stuff, then...
(ii) a change of circumstances since it was issued has removed the basis of the holder’s claim to admission, except where the change of circumstances amounts solely to the person becoming over age for entry in one of the categories contained in paragraphs 296-316 of these Rules since the issue of the entry clearance;
The exceptions they refer to as listed in paragraphs 296 - 316 have to do with children admitted with a view to settlement (e.g., the child of a person admitted as a fiance or spouse) and now want to apply for permanent residence along with the primary and hence do not apply to you.
And for port-side refusals, Paragraph 321A provides the reference...
321A. The following grounds for the cancellation of a person’s leave to enter or remain which is in force on his arrival in, or whilst he is outside, the United Kingdom apply;
(1) there has been such a change in the circumstances of that person’s case since the leave was given, that it should be cancelled;
So Paragraph 321 gives the Immigration Officer a mandatory grounds for your removal. The things listed in Paragraphs 320 and 321 are without discretionary latitude (i.e., the Immigration Officer has no choice in the matter).
What all of this amalgamates to is: don't do it, you could find yourself in detention facing removal.
Adding: in theory this should be picked up by the carrier who would then refuse boarding permission.