I can see no reason why you shouldn't be able to request repeats via email. I can see that there might be certain security concerns, but these could be addressed so easily I doubt that they would stop such a system in its tracks.
In the end it's up to the practice to decide on a protocol, although when the NHS's much-vaunted new IT system eventually sees the light of day I imagine that the use of email in GP practices will accelerate. Under the new system any doctor in Britain will be able to access the medical records of anyone on a GP register in the UK, and we'll all be able (eventually) to access our own medical files online. What fun it will be debugging that little lot - I imagine that the people who are working on developing the software are wondering if they've been handed a poisoned chalice.