I'm glad you're so easily impressed! The responses are much appreciated.
I have already discounted Green Motion and Drivalia.
The Excess Protection was for National Car Rental. I'm not using a broker; it is an online booking. This is what they say:
"Excess Protection (EP) is an optional coverage available only if Damage Waiver (DW) is included in the rate or purchased. EP reduces the applicable Damage Waiver excess amount. If EP is not included in the reservation, it is available for purchase.
For all Cars and SUVs of the categories Mini, Economy and Compact the excess can be reduced to 100 GBP. For all other vehicles the excess can be reduced to 250 GBP. Excess will be charged every time a vehicle is damaged, lost or stolen."
It's an extra £20 a day, but probably worth it.
Buying a booster cushion is a great idea, as getting it through the rental company is £15 a day+
They have confirmed I can use my Thai DL - not need for an IDP.