The English Department is located in Foster Court in the main campus. You basically go into UCL through the entrance on Torrington Place, and it's about halfway down. They have seminar rooms in the building, but whether your seminars will actually be there is another question entirely. UCL timetabling is a bit of a law unto itself and they seem to just send people wherever there is space.
I did History, and some of my seminars were held in the history department (at the back of the main campus fronting Gordon Square). But some were also in other departments - I particularly remember having to go to the Anthropology and Maths departments quite often over the three years.
Lectures are quite different because many departments don't have big enough lecture theatres to accommodate everyone. I had many of my lectures in the Chemistry, Biology and Engineering Departments. But you may find that some of your whole-year lectures are online if UCL is still following covid rules.
You will probably have to go further afield sometimes. I had one module based in a room that backed onto Great Ormond Street Hospital, and another that was near Kings Cross station. It's all pretty walkable though - you'll probably never have to go further than 10-ish minutes from the main site. Nearly all their buildings are within the Bloomsbury/Kings Cross/Holborn area, with the exception of UCL East near Stratford, but I doubt you'll have anything scheduled over that way if your course isn't specifically located there.