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The study gap for undergraduate and diploma students up to 2 years is allowed and for postgraduate candidates, the study gap up to 5 years is permitted. There is an 8-year study gap available in various institutions and universities. There would be no difficulty in getting a study visa after 8 years gap if you have been academically sound and have clear trajectory by way of employment or further studies. 

As soon as you can present your case properly and explain the reasons for a study gap, you should not find it difficult to obtain UK visa approval.

Most universities have different requirements for study gap. Some are strict while others are flexible. Gaps do affect you candidature but it can be backed up by the strong profile and personal statement/Statement of Purpose. You need to explain the reason for your gap. If you had any medical issues you can provide medical certificate along with other documents.

Generally, gap of 2 years is acceptable for undergraduate courses and 5 years for postgraduate courses but more than 5 years of gap is acceptable if you have been constantly working or engage in some other activity. We have seen that the universities accept anyone with a genuine intention to study regardless of educational gap.