Just stayed at the Gainsborough for 1 night a few nights back. It was a same day booking as our plans had changed. We booked through the Amex plat desk and got the same Amex offer.

Without Amex offer and booked direct
270 pounds room, deluxe room
free massage for 1
bottle of champagne
20% off spa
access to the Roman spa two hours in the early morning and two hours at night. Or anytime if you pay for a spa.

the offer we took through Amex plat desk:
270 pounds room. Upgraded through offer to a larger corner room.
dinner for 2. Unlike the review description above, it looks like we had a choice from the entire menu but capped to two courses per person. Our bill came to over 100 pounds but was not charged. Service charge was 0% but we left a tip.
hot breakfast for two. Continental was buffet style and the hot breakfast from menu and booked to order. It was all very good. Value: 30 pounds per person.
same access to spa as non Amex: we found that 8pm it was empty but by 9pm it is very busy- go early.

we looked across the road from our hotel and saw the thermae spa was also packed. we recommend you go at a off peak time for a relaxing experience.

we had no regrets taking the Amex offer vs the hotel offer.