Without knowing the ins and outs of the policies involved, im guessing it comes down to 'reasonable adjustments'?
If the restaurant in question can show that they have attempted to make adjustments to allow the party, but can't allow the full request due to safety/financial/health&safety reasons then its not discriminatory.
Eg my old workplace was a public building with no wheelchair access or disabled toilets. But this was not breaking any regulations because of the age of the building, and the cost it would take to nake the adjustments.
If the restaurant can show that they qould make substantial financial losses from giving the table at that time, but have offered an alternative rather than just saying no, then they are within regulations i would assume.