My understanding, from my Dad, is that for safety purposes, you have the rear brake on the left, to give controlled braking when turning right and indicating at the same time. If it was the front brake on the left, it could be too severe and you could easily go over the top. (I've done it). On the continent, the same logic, but brakes are switched round.
It's only too severe if you pull too hard...
In any case, you should probably be approaching turns using the same principle as when you're driving a car: mirror-signal-manoeuvre, with braking as part of the manoeuvre phase, ie
after signalling. To illustrate: the regular route I used to ride to work in my previous job involved turning right off a busy road on a steep descent. Having the front brake on the left when making that turn would have been far safer - using the rear brake alone while signalling wouldn't have given me enough stopping power. The way I usually approached it was to signal right well before the turn and move out to the centre of the road, then use both brakes as I got close to the turn.
And if it's safety you're interested in, you really ought to have
both hands on the bars when braking for maximum control. I discovered this recently when riding a borrowed bike with hydraulic disc brakes - hadn't been riding it long enough to get used to how bloody powerful the brakes were, applied the rear brake slightly too hard while riding one-handed and almost lost control.
Another bike I borrowed recently was set up with the front brake on the left. Only no one told me it was set up that way. I nearly found out the hard way when descending Fountain Drive (from Crystal Palace) - a lorry started to reverse out of a driveway ahead of me on the left, forcing me to brake hard. I didn't noticeably slow down so started to brake even harder, which just caused the rear end of the bike to start fishtailing all over the place. And I still wasn't slowing down. In the end, I had to do an emergency unclip and jam my foot down, completely wrecking my cleat. Luckily, I came to a halt in time though.
While I stood at the side of the road, I inspected the bike to work out wtf was wrong with it, and that's when it finally clicked. After that, it was fine - once I knew which way round the brakes were set up, I found it perfectly easy to use them that way. It didn't make any difference to the way I approached junctions or made hand signals.
A colleague of mine did find out the hard way that his borrowed bike was set up that way round. He made the discovery while taking a hairpin bend on a mountain descent in Mallorca... There's probably still a nice blood stain on the tarmac there as a warning to other riders. (Actually, I think he had already been told the bike was set up that way, he just forgot in the heat of the moment.)