"I take a 9:30 flight to London in the evening."
The English version of this sentence is awkward.
Furthermore, the Chinese as 我坐晚上九点半的飞机去伦敦 would be "I take a 9:30 evening flight to London" while 我晚上坐九点半的飞机去伦敦 would then be "In the evening, I take a 9:30 flight to London" or "I take a 9:30 flight to London in the evening"
But it doesn't matter because make the English sentence be, "This evening, I will take the 9:30 flight to London" unless this person is some super-business man who habitually takes flights to London often enough (and on a regular schedule) that this sentence would make sense.
晚上 and 九点半 go together in this sentence, which means the English translation given is incorrect. It should read either 9:30 p.m. or 9:30 in the evening, without the prepositional phrase "to London" spliced in the middle changing the meaning around. Good catch! (And, of course, that "a" should really be "the".)