Accepting the tenancy agreement online would be just the same as accepting and signing the agreement in person.
Unfortunately, the 14 day cooling off period that normally applies to online contracts and purchases, does not apply to tenancy agreement.
Therefore legally, you can be held liable for the tenancy agreement that you have entered into.
However, as you are outside of the country, it would be very difficult to enforce against you. The landlord, could start proceedings against you in court for the amount due.
They would first have to serve pre action protocol letters, then take the matter to court, then obtain a judgement against you, and then finally enforce that judgement. If they do this, you would end up with a CCJ against you which would flag up on credit agencies in the U.K.
Being an international student and living overseas, I would find it unusual if the landlord would spend their time pursuing you. It would be far quicker to just rent the property to a new tenant. But that does not mean they will not pursue you.