What to expext/ask during heat pump survey?

Evening, finally pushed the button on getting some quotes for a heat pump.

Octopus quoted £1841 after the BUS grant and are coming tomorrow morning.

To get a comparison I had a quote from Aira today after they came round. And a rough quote from a local supplier just over the phone.

Aira came in at £7214 after the grant for their All in One unit and 6kw heat pump.

The local installer has no availability to come round for a month but gave a quote of £11000 after the grant based on him doing a similar property around the corner.

So first question is how/why are Octopus so much cheaper (I know this may change slightly after the visit)?

And what should I be asking the assessor tomorrow? I didn't really know what to ask today!

The Aira lady didn't measure a single room or window, but based her calculations on the floor size on the EPC (89²m).

I did mention the Octopus online quote and she said they were always priced well but the Aira equipment is so much better and they guarantee a SCOP of 4 running at a flow temperature of 45C, saying Octopus always run theirs too hot at 55C. She also quoted for changing all radiators because they are single panels. Her quote was also for an all in 1 unit rather than separate tank. Is their stuff really £5373 better?

She also said my electric shower will stop working. Is this true?!