Needing a separate earth electrode for an outside installation is perfectly normal - but in most cases that's £10 of parts and maybe 10 mins to fit it. If they've quoted for a 'pit' and that means any more than the
little plastic cover you normally get to stop you tripping over it, maybe you have a hard-surface driveway that they are planning to make a more substantial hole in (with a cover like
this - but the labour being more significant). Normally you can avoid that by cabling round the corner and sticking it in a flowerbed instead.
The other thing you need for compliance with regulations is a double-pole type-A RCD; although Tesla's unit has an RCD function of some sort inside it, that one is self-resetting and doesn't appear to meet the regulatory requirement. In principle you could swap an existing RCD in the consumer unit for a type-A equivalent and then just use an MCB way in the consumer unit as you have in mind, though that's not ideal since any fault on the chargepoint would also take out all the other circuits sharing the RCD. Depending on the mix of other circuits and arrangement of RCDs in the existing setup, that might or might not be considered compliant. Most likely, they've quoted to provide a separate RCD external to the consumer unit.
I hope the £850 is including the Tesla WC itself - if it's just for installation it sounds well over the top.