http://en.wikipedia.org/wiki/Returning_officer
Usually a civil servant. No guarantee of avoiding bias, but generically most of them seem to be fairly civically minded.
I may have issues regarding the postal ballot system which was poorly designed and wide open to abuse but, by and large, our voting system is fair. keep to the Fen Causeway
But the postal ballot system has been proven to be fraudulent. It's wide open to abuse and lacks the safeguards that hand counting offers.
We'll surely see a push for e-voting soon, but given the UK's technological skills I wouldn't expect any progress on that for at least a couple of decades. By which time it probably won't be an issue any more.
People do sometimes ask canvassers to help them fill in the forms, if they can't see or read too well etc. We are not allowed to touch them. I can't remember if it was actual fraud or a very stupid mistake but you won't find Labour Party campaigners interfering with postal ballots now - if they do, they shouldn't be in the party, and would get kicked out. Ad astra per aspera
The problem with the US elections seems to be the complexity of the ballot, but that is an artifact of insisting on putting on the same ballot all elections and referenda being voted on. It'd be nice if the battle were only against the right wingers, not half of the left on top of that — François in Paris