Not done it myself, however, I do find going to a local indepent will be of more use as they probably have a tried and tested solution, as opposed to the opinion of someone who perhaps may have not performed the task at 17 years old.
Gaffer tape over the holes to stop the concrete filling them, easy enough to cut it away once the concrete goes off as long as you’ve supported the anchor so it doesn’t sink/float or tip over. Remember concrete is over twice as dense as water so an air filled tube will tend to float out of it.
Postcrete will be more expensive than knocking up concrete for a decent volume (work out the volume of hole you need and you’ll probably find you need 10 or 20 bags of postcrete) and it tends to go off in ten minutes so make sure you have everything in place before adding water.
This should help you out. Looks like you’ll need a fair amount of concrete so as said postcrete will be bloody expensive. Like the idea of putting some rods horizontally across the hole to make it more secure.
If you decide you want a bit of rebar, let me know. If you can collect it from Southwark, I can give you as much as you need. Probably not required for a Y anchor in a small lump of concrete unless you think someone is going to attack it with a jackhammer.
but still make sure it’s flush with the floor?