The value of gap year activities is widely documented and most universities and colleges will allow you to apply for deferred entry, but check with them before applying. When you apply, include details of your proposed gap year in your personal statement to support your application.
A note for international applicants:
Several countries require their citizens to complete mandatory military or national service. If this affects you, check with universities and colleges in advance to discuss when you should apply. You can defer through UCAS for a maximum of one year. It would be useful to include information on your military or national service in your personal statement.