For those saying he won't remember it don't you do anything with children until they are about 7?
I am one of the first to question people taking their children to Disney when they are very young, not because they won't remember it, but because when we took DS there were younger children there tired, grumpy and petrified of all the people in costumes, so it's not that they won't remember it but because they might not appreciate it.
It appears the OP's son is obsessed by Egypt and so he might not remember the holiday when he is older, but sounds like he will really appreciate it and mum gets to see him so excited about it. I think if I was any of OP's relative who had given money (for saving or gifts) I would be looking forward to the 4yos tales of adventure and pictures of his happy face in front of the pyramids. You would have to be a real curmudgeon if you begrudged the OP spending your £100 birthday gift for the little boy to spend on what will probably be the most exciting adventure he has ever been on.
The OP is not wealthy, she is being careful and hoping to save £18k for her son, which is more than many children will be getting on their 18th birthday. If her son wasn't obsessed by Egypt it sounds like this would not be the OP's holiday destination of choice, so she is doing it for him.
Posters on here who give money for birthday presents and who give it directly to parents when the children are younger and expect it to go in savings accounts, what do you do when the children are older, give it direct to the child and tell them to save it or let them spend it on what they want.
If relatives are so precious about what the money is spent on, they should set up their own savings account for the child to be transferred to them on their 18th birthday