I have a one-year visa to study in Sweden ~ Do I need a visa to go to the UK for three months?

If you want a visa, you can apply to the British Embassy in Sweden. It's easy. The general requirement is to have a visa in Sweden for more than half a year, and to improve the proof of funds. A three-month bank statement with thousands of euros on it is almost enough.

And it depends on whether you are visiting relatives or traveling.

If you want to visit relatives, you need an invitation letter and proof of accommodation (the hotel and a friend's landlord have your proof of accommodation).

If you want to travel, prepare your own travel plan.

It's simple.