This is a very serious problem.
As the guidance notes, you must actually be travelling with the adult named on your UK entry clearance. It is not sufficient to be joining them, even at the airport. (If this were sufficient, it would say so explicitly.)
So, you will probably be denied boarding your flight, as your mom won't be with you when you check in at the airline counter.
Things get worse if you somehow do get on the flight and arrive at the UK border. Since you don't meet the conditions of the visa, it is possible that you may be refused entry and returned to your point of origin, not to mention have your visa cancelled.
If your mom actually is waiting in the airport, though, it is likely that both of you will be having a very long and uncomfortable chat with immigration officers. And it's possible that at the end of the chat both of you may be leaving the country on the next available flight. It's also possible that you will be granted leave to enter or a temporary admission, but which way it goes is something I can't possibly predict. Much of it will depend on what your mom has to say about why she did not travel to the UK with you as your visa required.