I've used SLRs for years, the great advantage was that the picture would be what you saw through the viewfinder, particularly as the metering was throguh the lens. The size of SLRs means large siameter lenses which gather a lot of light meaning pictures in poor light conditions. Much has changed when cameras went digital, light levels are not so important as the digital sensor can be made to work in low light conditions. Increasingly even SLRs have real-time screens on teh back so that many people now don't even look through a viewfinder, they look a tthe screen to see what they're going to get. Lens interchangeability is still an advantage with SLRs, because zooms witha wide range are a significant compromise compared to several lenses covering the same range of focal lengths.
I currently use a Nikon EOS 3100 and Nikon D100, and I also have a film Nikon F2. I use a fisheye lens with the D100 for stereographic images for high hedges cases, but rarely change lenses thereafter. It is more important to have a wide angle lens than a telehoto, because you can zoom in later digitally but can't re-create a wider view once you've left the scene.
The modern SLRs have anti-vibration gadgetry, which works noticeably well, but I would say that one of the best things about SLRs is that they are heavy and therefore don't shake easily. Lightweight cameras are great for portability but you're more likely to get blurred pictures.
And I ile that if one bit of an SLR breaks, it can be replaced. Not so with other cameras. My D100 lens is playing up, so I can keep the body and am shopping around for a second-hand replacement lens. All my filters and lens hoods will fit the replacement lens. I like that, indeed it drives me nuts when good kit has to eb chucked out because one little bit of plastic snaps off.
A word of warnig abut Nikon. As someone else said, they are trading off their name - their more recent lenses are plastic and don't last. The D3100 was barely a year old when the aotofocus started to stick. The D100 lens no longer works on manual focus, which drives me crazy since I can't be bothered with autofocus (try taking a picture of a leaf and it focuses on the sky behind). I am on eBay a fair bit trying to get e new lens, and the 18-55mms that come with the current Nikons are a dime a dozen, all being ditched by their owners I suspect because they are starting to show signs of failing autofocus. You can get a modern 18-55 for £30, but one that is 10 years old, back when Nikon still meant quality and lenses were still made of metal, costs you £200 second-hand.
To be honest, for work I would like a bridge camera, the zoom ranges are incredible and they are pretty compact without being too flimsy. But I will possibly be the last person still using a SLR because for versatility, creativity, complete control and almost endless expansion possibilities they can't yet be beat. But can I recommend a current SLR for £300 for someone whose going to explore photography with it? I don't think I can. My wife got her Nikon as her model of the 3100 was being discontinued, and it was discounted and was a bargain, but I would be wary unless I had read reports from a non-partisan magazine.
There used to be dozens of mainstream SLR makers, now it's just Nikon and Canon, in an ever-diminishing market. They are cutting corners. Only their pro and semi-pro stuff seems to be any use, as it is made for people who aren't tryin g to choose between SLR and other formats. Poeple for whom control is more important than convenience. Quality of pictures for point-and-shoot punters is hardly the issue any more. An iPhone can compete with an SLR now.
So in my view it depends what kind of person you are buying for. I think £300 will just about get a reliable SLR.