Farringdon to LCY dep 0700 about 30 mins cost about GBP27 plus tip.
Ditto dep 0800 45 mins; maybe 1 hr, although you are going the opposite way to the heavy traffic.
Fenchurch St will be shorter
If your flight is 0930 suggest you head there early and enjoy a nice leisurly cup of Nero's coffee and a pastry while reading the paper.