In most cases, they are the same thing. Wych elm is one species of elm and these days it is the predominant one, the other species either being quite rare or having been decimated by Dutch Elm disease (e.g. English Elm, Dutch Elm, etc) so the vast majority of Burr Elm we see is in fact Burr Wych elm.
Jim is correct, the Wych elm is mainly found in the north of Britain.