I have no idea about the political culture in the UK. Is it possible that he just didn't have as much data to work with? In other words, a lot fewer relevant polls to feed into a model?
we don't generally have very many regional polls, and certainly not constituency polls, that are available for aggregators (an internal poll in a key marginal, perhaps.) There's a lot less money spent on UK elections.
His US model makes a lot of allowances for things like how self-proclaimed independents are likely to vote.
It didn't work as well in the UK though.