This article “Could e-voting be on its way in the Uk?” would also be good news for British expats if implemented:

“Campaigners for electronic or e-voting say “digital democracy” is on the way in the UK. Although next year’s general election will be run using the traditional system, they predict a different landscape by 2020. “

British expats not already excluded by the 15-year-rule  are already encouraged to register on-line to vote on the government website www.gov.uk/register-to-vote.

However, without the convenience and encouragement of then actually being able to also   e-vote on-line, the overseas voter is still faced with the problem of missing the election date deadline due to the delays inherent in the postal vote system, or finding a suitable UK resident to submit a proxy vote.

These are also reasons, including the 15-year-rule,  for low voting rates among British citizens resident abroad.