Food aggression in a spaniel

Izzy is nearly 3 now and recently I have noticed a escalation in Izzy's problems with other dogs, especially around food. She will go to tell another dog off if they try to eat her food/ chew. She may also do this with toys.

This has gotten much worse over the last few months, even before she was bitten by another dog for a ball, then got herself info a fight (no blood drawn). I would say that she is ball obsessive, but not indoors.

However I am worried about the progression in her behaviour. She is getting more and more nervous and distant with other dogs.
Today she told the puppy we are looking after off (quite nastily) for going for "her" bone (she had both his and hers). She hasn't done this before when we look after him.

A further note is that I have told my parents (she is the family dog) of her problems and that she needed socialization, but they either brush me off or tell me it can wait! They do not seem to listen to me, even though I trained her and know her normal behaviour.

I am just worried about Izzy at the end of the day, so I am looking for help in managing and hopefully solving her problems...

I'm not quite clear on your living situation there. I know that in a multi-dog house you want to feed everybody in their own bowl at a distance from anyone else. Even cats will get testy over their food bowl but they often work it out and take turns.
Even dogs who are quite friendly with each other normally, need privacy while they're eating. We have a big kitchen and the dogs each have their own food and water bowls on opposite sides of the room. The girl can be obnoxious about stealing tasty morsels. But we watch to make sure she stays at her bowl.

Yes, the dogs need space to eat alone. My Nigredo will even let our cats eat from his bowl and doesn't mind if our other dog Jade comes near him. Jade will badly snarl if he comes near her while eating, though. She has snapped at cats.

They give up instantly and run. If Nigredo persists, she will snatch up the morsel and run if she can, but if she can't, he will steal it. I always supervise now to prevent this from happening. Food guarding is common amongst dogs and normal.

She might be snarky because she feels constantly threatened by other dogs. You've really got to make sure nobody bothers her while she is eating. Let her have her own toys, if she doesn't want other dogs bothering her, don't force her into those situations. My Jade would cause me tons of problems if I forced her into things she doesn't like, but as it stands, she's no problem at all. I avoid things she doesn't like.

Dizzy can certainly use some socialization, but it needs to be done the right way. Gentle introduction with balanced dogs is helpful at first. Not sure how old you are, but you can begin some of this yourself most likely. Lots of reading to be done about canine socialization. Take a Google whirl and see what you can find.

Well I would ensure she's fed seperately for the other dogs protection and safety. I normally train mine to eat together but clearly she is both stressed and aggressive. I would ensure she has a solid "Leave it" command and use it whenever she attempts to nick anything that isn't hers. I'm guessing the ball belonged to the other dog but it's not clear. I would train her to stay fairly close on walks if she can't be trusted around other dogs and balls etc. I wouldn't tolerate this behaviour at all, either inside or out and I have easy going dogs but if she went for one of mine over a ball I wouldn't be letting you or your dog off lightly.

Think of it this way, if someone tried to steal half your dinner while you were eating, you'd probably get mad too

Dogs really should have their own space to eat in peace if you want to keep harmony

You have been given some great advice but one vital thing that I'm concerned has not been mentioned.
You have said how this is escalating, just on that point alone she needs a full vet check up. I'd get them to do a full body check up, feeling joints, checking teeth, eyes, ears, paws, blood work, make sure she is in perfect health. Then once you have the all clear you can start on changing her behaviour. If there is something making her feel ill, in pain or more anxious (loss of any senses) then you need to deal with that first.

Fed her in another room. Don't leave any bones down, give each dog one, let them have it for so long and then take the bones off the dogs. And don't leave the dogs alone while eating.
Both of mine will growl, snap, show teeth or tell each other off over food. It hardly ever happens as we are careful. Just think if someone tried to take your food you would likely react as well.
I make sure my dogs are left alone by both dogs and humans. I would not like someone trying to take my food away so I don't do it to my dogs.