I was talking about breakfast at Hyatt Erawan (1.5 hrs), then prayers at Erawan Shine (as long as it takes you). Both locations are open at 8am.

I forgotten to mention. I did take an "Airbus" at the lower ground floor of the airport before (costing B150) and that takes you to BigC/ Word Trade. However given the lighter traffic in the morning, the fare diff between that and taxi is really worth it in my point of view. But yes, if you take a local bus then it will be a lot cheaper.

Last time i checked for C to F upgrade at LHR check-in was $750 USD but that was over a year ago.

You can also check with the airport Novetel for a 4 hrs short stay or just to use the gym/pool facillities. If so, pls let me know. I would like to know how much it would cost ^

Cheers