The big thing to watch for is dehydration. Try to get water in her little and often and ensure she has wet nappies. If she goes 24 hours without a wet nappy, they will take it seriously here. But before then... not so much.
Try not to worry about food right now. Keeping her hydrated is the most important. Milk can be tough on a delicate tummy so maybe try water at first.
Yes, milk is tough on little tummies because it's harder to digest. If you don't have Pedialyte, try Gatorade "G2" or equivalent because this has less sugar. Also, don't give any red/orange/yellow colored Gatorade G2 because these might affect the color of urine/stool when they're trying to look for any blood. So stick to the white or blue colored ones.
Below is the list our hospital nurse gave to us. However we are in the US, so I don't know what the guidelines are for the UK.
When To Call: 3 Months to 3 Years
Call the Appointment and Advice line if your child has a fever of 100.4°F or higher and any of the symptoms or signs listed below:
Trouble breathing
Wheezing or tightness in the chest
A deep cough with lots of mucus
Vomiting or diarrhea
Signs of being dehydrated (refuses to drink; has a dry mouth or cola-colored urine)
Severe headache
Stiff neck
Very sore throat or trouble swallowing
Skin rash
A bulging soft spot on his or her head
Severe ear pain
Painful joint with swelling or redness
Pain when urinating or urine that has a foul odor
Irritable or crying constantly
Does not want to be held or comforted
Seems confused or extremely sleepy
Lacks energy or is limp or listless
Also call if your child is not improving, seems to get better and then gets worse again, or has a fever that doesn't come down after 3 days of home treatment.