Would this be being a rude knobber or not?

[NeedsAsockamnesty]

If I was going to a restaurant (where I eat regularly and am known quite well) on Christmas Day for a meal with some of my children on that day is serving a 4 choice set menu for each section of the meal but on normal days serves a normal type menu,

How UR if at all would I be to ask them very nicely with lots of notice (and it would be an ask not a order) if they could allow one of the children (adult price) to advance order from the normal menu instead of from the set menu.

The normal menu is much cheaper than the set one and I would be happy to pay for the normal menu price on top of the price for the set menu for him so they do not lose out at all.

The child needing the change has autism and is significantly effected by food related sensory issues.

So unreasonable or not?

requiring them to make a different meal to the set one wouldn't come under that regulation

Why on earth wouldn't it? It'd be a perfectly "reasonable adjustment", which is what the law says is required. Asking for golden cutlery, boots polished while you eat, £300 worth of specific food requested five minutes before the meal is served would not be "reasonable". But asking well in advance for a dish that they do normally make, because a child is restricted in what they are able to eat? Why do you think that's unreasonable?

I've eaten out on Christmas Day at a new restaurant before. DS is super fussy. They had no issue making off the menu, Christmas spaghetti Bolognese :P

Ask them! Sure they will be ok with it :)

Yanbu, no harm in asking, and what you are asking I'd hope they'd be fine with.