According to recent figures from the UK Electoral Commission. Late voter registration was very high last weekend.
Yesterday BBC radio 4 discussed this. Today reporter Sanchia Berg stated that after the first leaders' debate 120,000 voter registration forms were downloaded from the Electoral Commission website. Jennie Watson, chair of the Electoral Commission also contributed with her perspective. Their comments can be heard online from the website.
Also interesting is recent report from Hitwise which discussed how the internet plays a key role in driving voter registration. It noted an increase in searching following the election announcement
Another factor may have been the use of social media advertising by the Electoral Commission which placed adverts on Facebook last weekend.
For examples of its advertising materials. Take a look at the official Electoral Commission website Other organisations which also seek to encourage new voters are Rock the Vote and Operation Black Vote
For historic data on levels of registered and unregistered voters in the UK a good starting point is the research section of the Electoral Commission website. For instance see this recent report on the completeness and accuracy of the electoral registers in the UK.
Another good place is the Office of National Statistics. Again search for elections. Examples of data include: Post-war turnout for general elections, 1945-2001: Social Trends 33 and Parliamentary electors on the 2000 and 2001 electoral registers
International Idea also has an excellent website which contains statistics and reports on voter turnout worldwide. You can use it to compare the position of the UK with other countries.