Tried my SPG AmEx card for one GBP charge--£17.50 posted as $23.79 last night ($1.3594 to £1). The highest GBP has been on xe.com since the charge authorized on September 24th was $1.3541, indicating a 0.4% premium at best.
Also, another Visa charge on the same date (and which posted at the same time) works out to be $1.3572 to £1. Looks like MC is currently better than AmEx which is better than Visa for GBP, though the percentages are still low enough that it probably won't matter much for bonus spend or most casual travelers.
BTW, props to AmEx for texting the foreign currency amount in their notifications instead of just the converted USD amount as with most other issuers. Unfortunately, foreign charges don't seem to appear in the pending transaction list on their website; not sure why that is.