Once you have received decisions from all of the institutions you have applied to, you will need to choose which institutions/courses you wish to make your firm and insurance choices and which offers you will be declining.
Information regarding the deadline by which you have to respond to your offer can be found via the link below:
Replying to your offers
If you do not respond to your offers by the relevant deadline given above, then your offer(s) will be declined by default by UCAS. You would then have 14 days in which to contact UCAS to ask them to reinstate your offer(s). Unfortunately LSE will not be able to reinstate offers once the 14 day deadline has passed.