It depends on what route you fly and how far in advance you book. That said, for many popular routes where there is competition you can often fly for 40-80 GBP rountrip if you book ahead.

Skyscanner will be a great resource for you. Just note the baggage and other fees on the low cost airlines, as well as which airports they use on a particular route. Getting to and from London area airports, Easybus might also be a good resource for you.

To get over to Paris, Belgium, and Amsterdam, check out Eurostar (high speed train) for youth and student offers. This will be faster than flights, when you consider airport transfers, checkin and security waits.

Finally—and you didn’t specifically ask—within England, Scotland, and Wales, check out Megabus sometimes with specials from 1,50. And don’t settle for National Rail prices; compare with Virgin Trains. More tips and advice for UK-based travels here.