I have been fortunate enough to have some good luck with Ancestry 's database of Electoral registers from the U.K. which has some search options. The FMP records are not so easy to search but there is this map of what records they have available. Since they are PDF files there are search options available within the individual pages via Adobe. The best tip FMP gives is this I think. "Many of the registers will have a street index or directory at the beginning of the document. You can either get to the beginning by continuously clicking the previous button on the PDF screen or you can start a new search for the constituency and year and then order the results by image number. By doing so, the top image would be the first page and you could then click through the images from the beginning."
The map has been immediately useful as I could zoom in and see large areas of my counties of interest are not included in the records
That's what I thought Charlotte and FMP ought to have made it clearer I think. Ancestry's weakness is not always getting the right page attached to your search but it's normally only one page forward or back. What I like with their collection is that I can put in a street as a keyword when all else seems to fail.
That may or may not be the case - the boundaries of the divisions aren't shown. For example, if you zoom in on west Essex, the heavily-populated area between Hackney and Romford has no markers at all, so you might assume there is no coverage. However there are entries for Ilford, Leyton, Forest Gate etc (they all come under Romford).