Trying to answer quickly as you are trying to make the booking.
Doesn't look like cash + miles to me as £409.79 charge says taxes and surcharges only. No mention of fare. Also, 85,000 seems more like the amount required for an AWARDS TICKET.
I'm not be any means certain of this though. Below link says you can only redeem between 2,500 and 50,000 points. Also dependent on what fare class you are booking (which I can't see from above).
You need to check whether you are definitely booking via the cash + miles option
https://www.thaiairways.com/en_TH/rop/Redeeming/CashPlusMiles_onTHAI.page?