When travelling by plane, getting a taxi to the airport can cost big bucks.
This is especially the case for the airports used by budget airlines (Easyjet, Ryan Air etc) that tend to be a long distance from the city center. It could cost you anywhere up to GBP100!
Public transport is often a far better option, especially for solo and/or budget travelers.
Before booking a flight, if you plan on using public transport to get to/from the airport make sure the public transportation will be running at the hour/day you need to get you to the airport on time.
Public transport options might not run as early/late as you need,
or might be more limited on weekends or public holidays compared to regular weekdays. Make sure you check this. Even taxis may be very limited and/or charging inflated prices on public holidays.
You’ll need to allow lots of extra time in case the transport is not running on time etc. I’ve had experiences of airport buses being full by the time they go to the stop I was waiting at, the driver informing us he wasn’t taking any more passengers, and needing to wait for the next bus.
If you’ll be carrying bags, also consider whether you’ll need to be walking up/down stairs etc if taking public transport.
Its sounds like there is a lot to think about but its worth it!