Maybe thats why they've only replaced a section 'i think if whole floor was replaced building control are involved, and then as you say insulation would be required.
Around £1500 would easily cover the work their going to do, £13,500 is way overpriced for the full floor, looks like they added around £10,000 extra, think you need to get rid of these conmen.
As above, you are being ripped off big time, what are the pipes in floor? and are they just going to concrete over them, 3 £15 tins of bitumen looks like their total damp proofing work.
On the Ryobi website, its available on offer for £99 with battery and charger, add this to your basket then just choose your free tool.
https://uk.ryobitools.eu/batteries-and-chargers/18v-one-batteries-and-battery-chargers/18v-one-battery-charger-kits/rc18120a/rc18120a-140x/
Normally the OSB floor would help to square it, but you're putting that in after walls i think? 10mm is fine over that distance, it wouldn't even be noticed, but you could set the walls a few mill in/out to get back to square.
Sounds to me like a Toyota Aygo/Fiat 500 type car would be ideal, reliable, cheap insurance, low tax, good on fuel, parts readily available etc, not a classic but something that will do all the things you need it to.