Do you already know the institution you wish to enroll in? They sure have their preferences posted on their websites. If it's the UK, they will surely prefer IELTS. Also note that there are two versions of IELTS, Academic and General. For study purposes, you should take the Academic module.
If you decide to go with TOEFL though, they will surely have their minimum score for admission in the UK institution as well since TOEFL is a widely accepted and recognized test.
If you do not mind which one you take, I would suggest IELTS for the UK.
Regards,
Jaime