Just had a look at the timings and surprised me that despite the backtrack, CX gets you there quicker. i.e. quickest connection:

CX 270 AMSHKG 1310 #0620
CX 705 HKGBKK 0800 1005
vs
BA 431 AMSLHR 1135 1155
BA 9 LHRBKK 1600 #0940

Anyway, my answer is that i'd fly BA and use the £350 to book another economy flight to Bangkok later in the year!