They have already required her to get a prescription for the calpol he brings into school so that they can be required to administer it when he needs it, now they are being fairly obstructive again
The calpol thing. The person who administers the medicine is legally responsible for doing so. If the child suffered as a consequence, the person administering the medicine would be held liable.
Therefore, in order to safeguard staff, procedures have been put in place that they are only to administer medicine or pain relief that has been prescribed. The adult can read the child's name and the dosage and be sure that they are following correct procedure.
Otherwise, parents could bring in any old medicine and say their child is to be given it 3 or 4 times a day, or whatever. That is not good enough. So the school are not being 'obstructive' they are putting safety procedures into place.
schools is smug and thinks it knows better than the mother whom it has rather patronisingly fought every step of the way
Where are you getting this idea from? What, exactly, has she fought all the way? There are a lot of conjectures going on based on very little fact. OP states that she asked for him to drink squash. They said no. She insisted. They said no. She wrote a letter. They said no.
It's quite clear that the OP has two choices, either get a letter from the gp prescribing squash, or teach her son to drink water. She can do this at home. She can put rewards and sanctions in place.
Hi son, did you drink your water today?
No, I don't like it.
Ok, well you know the consequences, no tv/xbox/ds/wii/phone (delete as appropriate) for you my lad.
Awwwww but Muuuuuuuuuuuuuuuum...
Now, now, son, remember, it's your choice, your consequences.
If she did this consistently she would have it cracked very quickly.
Hi son, did you drink your water today?
Yep, I didn't like it but I just drank it down quick.
Fantastic! You have earned yourself a reward 
Yay! 