Surely allowing online voting would be a massive shakeup to everything?
It'd be a great idea, and it works for Estonia. Now there would be a whole host of security issues to look at and overcome, but I don't think it'd be impossible to do so.
It'd probably be a better start to change our electoral system away from first past the post though. In our current system, you could change who forms the UK government just by switching a few thousand votes around, targeting key marginals. That'd be a very tempting target to interfere with. But switch to PR and adding a few thousand votes to Party A's total isn't going to change a damn thing.