If you are a non-visa national seeking entry to the United Kingdom for three months or less, and are not coming via Ireland, you can apply for leave to enter at the border. If you want to do this, you will require a Certificate of Sponsorship, but will not be required to obtain entry clearance. Separate arrangements, consistent with the principles of sponsorship, will enable appropriate checks to be made at the port of entry. You will need to tell the Immigration Officer your Certificate of Sponsorship number when you arrive in the United Kingdom. The Immigration Officer will also ask you to provide your evidence of meeting the maintenance requirements. It may take the Immigration Officer some time to process the application. If you do not bring all the necessary evidence, it may result in your application being refused and you will not be allowed to enter the United Kingdom.
Where you are granted leave to enter for up to three months by an Immigration Officer on arrival in the UK, your leave will automatically lapse once you travel outside the Common Travel Area (the Common Travel Area encompasses the UK, Republic of Ireland, the Isle of Man, Jersey and Guernsey). This means that you will not automatically be able to re-enter the UK on the basis of your original grant of leave. However, you may need to come back to the UK to fulfil your engagements for your sponsor here. In this instance, you must tell the Immigration Officer your original Certificate of Sponsorship number on arrival. The Immigration Officer will ask you to provide your evidence of meeting the maintenance requirements again. The Immigration Officer will carry out checks to ensure that the sponsor has not withdrawn their sponsorship of you since your original entry, and may wish to speak to the sponsor again to confirm the details on the Certificate of Sponsorship. Provided you meet all the criteria, the Immigration Officer will be able to grant leave to allow you to complete the engagements for your sponsor within the period of your original grant of leave.