i hear sheffield is better on the engineering front. ive visited it and i think its a great department and university. however, i cant really comment on UCL, never been. its possible the grade reqs are just out of oversubscription!
if you can, visit both maybe? look at everything, not just prestige/academia. there may be some dicriminatory factors important to you specifically, that will help you to choose.
im a northerner, so on bias, SHEFFIELD.
i was going to apply there, but i thought it better to have 3 architecture choices instead.