You may be interested in a new TNA initiative: Digital Microfilm. It allows you to search and download some of The National Archives’ most popular records, which were previously available on microfilm. Many of the records are indexes and these may be helpful in locating other relevant records. These records have not been indexed and so you will need to scroll through the pdfs, much as you would when using a microfilm.
The pdfs cover records from the mid-17th century to World War Two and include, amongst others, Home Office, Foreign Office, Admiralty, Cabinet and War Office papers.
This is also bookmarked on HFL Delicious pages.