This makes so much sense!
I'm only getting used to commuting and tickets etc, so thank you all for answering (what seems to be) a simple enough question!
I didn't even think about Luton being past the furthest zones, stupidly!
All your help has been much appreciated, so I'm going to get an Anytime Day Travelcard from Luton to London Zones 1-6 four times a week, which costs around £23.30 per day.
I have 30 weeks of university so I'm estimating to spend around £3,500 on travel, which is a little more than half of my university maintenance loan... I guess that's what the money is for in the first place!
Thank you all