Oh absolutely sip sip sip, not full mouthfuls, sorry didn't realise you were so soon out - anything you have must be able to go up a straw, nothing thicker! Soups with marvel or cheese added, sieved to remove lumps, we were told we could have runny NAS custard - sainsburys do sachets of instant NAS also aunt Bessie's do one but make sure it is the NAS one!
If cold milk doesn't go well then try it warmed slightly, sometimes cold things don't go well. Also try thinned down yogurt - total Greek is good as high in protein but you need to thin it with milk.
Readybrek we could do but very runny - again it should be able to almost go up a straw.
Don't try purée or soft foods yet - at least 2 weeks on liquids but I would stick at that stage until you tolerate it.
Keep volumes small - I got a childs plastic beaker that held 200ml and it would take me a 1-2 hours to get that down to start with. Please be careful with dehydration, as
@starryeyed said you really need to make sure you are hydrated otherwise it can cause serious problems. I wrote everything down and how long it took me to drink it as well as what time I started. I recorded all volumes as well so I could see how much I had had and if I was managing to increase the volume over the course of a week.
Let us know how you got on with the milk.