It's certainly can be dangerous. A friend was taking a picture of us cycling behind him with his phone. He wasn't looking where he was going and if it had not been for our cries he would have been in a Dutch canalled.
But is it legal? The official answer is:
Using a hand held mobile phone whilst cycling is not illegal per se. However, you could commit an offence of careless cycling. It is also not advisable for the obvious safety reasons.
For those people using electrically assisted pedal cycles it depends on the cycle itself as to whether it is illegal or not. A lot of new vehicles are being marketed as electrically assisted pedal cycles but are, in fact, classed as motor vehicles so using a hand held mobile phone whilst riding one of these would be illegal.
However this hasn't stopped Police in West London fining cyclists for using mobiles – but say no resources to target drivers using handheld phones at wheel. Read more and be inflamed by the comments:
MEANWHILE ACROSS THE POND