Originally Posted by
RichardAtCT
If it is the usual 3 hours before
What is this 'usual 3 hours' you speak of? The only reason to arrive 3 hours before any flight is if you want to maximise your lounge time.
For an 0625 flight with a bag, your cutoff is 45mins. Aiming to arrive at checkin 0515-0530 would be fine.
That's too early for the Tube unless it's a Saturday/Sunday. Where are you coming from in London and what day?