https://www.indra.co.uk/
The above company is in Malvern. I get the impression they experiment with all sorts of EV vehicles, and might be able to advise on what could get damaged through water ingress, and if it could be repaired, as opposed to replaced as per the Toyota quote.
I was thinking along the lines of: how is the battery damaged? Is it just corrosion that could be cleaned off, leaving the battery serviceable? The dealer has to guarantee the repair, so replacement of parts, if they are in doubt, removes some of their risks. But you then get suffer a big bill that has covered any 'worst case' that they might come across.
I would have thought Toyota would design the battery so that it would survive a water leak into the boot, as I would expect the connectors to be at the upper part of the module (I've never studied one close up though!).
According to this site:- https://www.toyota.co.uk/hybrid/hybrid-faq/batteries the new battery, unfitted, is from £1140 including VAT, There is a hefty rebate given by Toyota on return of the old battery, I imagine the price given in the link included that, I wonder if your quote does?
This Northampton company might also be able to give some advice on likely costs and repairs, as hybrid batteries are their speciality:- http://www.hybridbatterysolutions.co.uk/
I have no personal experience of any these companies, I should add.