Just clarifying Graham's post and helping to explain Ralph's for Joan's benefit.
There are two groups - a large panel (130+ as described by Graham, names withheld), and a small committee (less than a dozen, mainly BBC suits, named).
The panel vote on the open awards, in the manner Graham explains, and the committee vet the panel process, and chose the closed awards.
The panel stage is very flabby. Graham's probably right about people making random choices (panellists have told me that often they haven't got a clue who to vote for), and the occasional anomalies can be explained by the fact that the full list of nominees runs to many hundreds, so very few people get into double figures. Best Artist could be up there with as few as a dozen votes out of 130 (The White Hare only got 8) - and there are scores of un-nominated runners up (I've even been one myself - which just goes to show how flat and wide the pancake is). Smoops say this guarantees a good wide trawl, but I think the cone needs to be sharpened somehow, because the figures would look a bit silly if they were announced along with the winners.
It's been suggested that artist managements might do back-scratching deals, but it's probably not worth their trouble, so I doubt there's much of it, if any.
I would guess (not because I know, but because it would be sensible) that the committee would choose between any tied nominations. Whether any other titivating of the panel awards takes place at the vetting stage is anyone's guess. I've suggested that the committee should publicly claim a right to veto anyone who's won recently, or who has been accidentally nominated in the wrong category (duo/group for example).
The committee awards are then to ensure a balanced, successful event.
You can apply to be in the panel, and it does contain a lot of reviewers. But none who are also artists - because we're excluded. (An agents may no longer vote for their own artists).
A bigger panel with narrower entry qualifications would be an improvement, as would a smaller mercury-style panel.
A fully public vote would probably return Mumford and Sons for Best Act every year along with a lot of young singer-songers who no-one here has ever heard of.