In order to carry out your academic career in UK with a non-student visa following measures must be kept in mind. You are not allowed to study with a General or Business Visitor permission. You have to return to your home in order to switch from general visitor visa to student category. But it is possible to get the permission to study with a multiple entry General Visitor visa or multiple entry Business Visitor visa.
You can study in UK with Tier 1 visa as long as you show working as your main purpose. In case you have to stop working it is advised that you shift from Tier 1 to Tier 4 (General) Students Visa. The immigration officers will cancel your stay if you stop working with a Tier 1 visa.
You can continue your studies under Tier 2 Skilled Worker as long as you keep working for your employer. You will not be allowed to further your studies once you have left working even if there is time left on your Tier 2 Skilled Worker visa.
With a dependent visa and clearance from immigration office you can study in UK. But if your visa expires and you cannot stretch your stay as a dependant you have to shift to Tier 4 only from your homeland outside.
As a diplomat you can carry out your studies as long as you are exempted from immigration control. In case you are working outside the country you can enter using your diplomatic passport but you have to obtain permission from immigration office to study.
For part-time studies there is no restriction once you have a non-student visa that allows you to study. But if the visa expires midway and you have applied for Tier 4 you cannot continue your part time study.