No, you can’t register a company with a student visa. It will definitely violate the policy. Another example, if you want to sell things on EBay, it is fine. However if you are buying a whole lots of product and sell it on EBay like all register sellers, it is considered as trading, which is for business. Then it is a no no for student visa as well. International students need to be real careful about it and seek advice because ukgov can just kick you out of uni and uk by cancelling your student visa if they find out about all that.