I suggest you contact a solicitor that deals with fare evasion asap. Just type 'fare evasion solicitor' into google. You're likely to be prosecuted for your actions, and likely to receive a criminal record. This may indeed prevent you from entering the UK and from finding a job if you were successful in re-entering. It is perhaps also best to alert your aunt as to your actions as soon as possible. If she knows now, and has time to cool down, a letter of good character from her is a very useful tool, as it was her card you used. If you tell her closer to the date, she may be less likely to write the letter if she's angry. I do suggest that you contact a solicitor as soon as possible on this case though. Depending on the circumstances using one may cost you about the same amount the fine would, and would prevent you from getting a criminal record. Best of luck.