I have now attained the dizzy heights of Royal Orchid Silver, and BKK check in put Priority tags on my baggage, on my return flight I left the tag on and the baggage check in staff informed me at LHR that the Priority scheme was not valid at Heathrow.
From the THAI website on Silver benefits when travelling on THAI (not other *A):
Exclusively on THAI
3,000 Threshold Bonus Miles each time Silver status is attained or retained.
Priority Baggage Handling
Additional Baggage Allowance - 10 kgs. (except when the piece concept applies and may not be available at all THAI destinations)