I can't speak for everyone, but I saw plenty of 18-rated movies when I was a kid. Robocop seems to be the one that everyone of my generation had to see, but you can probably put films like Predator, Terminator and Aliens in that bracket too. I don't want to get into a discussion about whether those films scarred me (I had a few nightmares about Predator, but other than that I'm pretty well adjusted), but instead look at just how we, as kids, were able to watch those films. The BBFC did its job, giving them a legally enforceable 18 rating, our parents weren't in the dark about movies (they'd been around for years), yet everyone wanted to be Murphy in the playground.
Of course, this is all just a roundabout way of saying that kids will get hold of and play 18-rated games. The Byron Review, which has been pretty universally praised for being both fair and making a lot of sense, should result in all games receiving BBFC ratings (something only a small percentage receive right now) and suggests that parents need to be educated about video games. But just as a whole generation of kids from the 80s managed to watch high-profile violent movies of the time, modern kids will play violent video games. I'm not sure what the government can do about it.
From personal experience, retailers selling violent games to kids isn't the real problem. Most modern games that feature excessive adult content are clearly marked with a BBFC rating (a few do slip through the net with just the guiding PEGI ratings), and every retailer I worked at during my younger years took those very seriously. Anyone who looked underage was asked for age verification ID and we frequently turned kids away after they'd approached the counter with GTA or Manhunt (the game all the kids wanted, no doubt due to the media circus being conducted in the mainstream press).
Retailers then, for the most part, are doing their jobs. The problem many have is the follow-up to the refusal to sell a legally rated video game to a child. In most cases the disgruntled kid would return a few minutes later, parent in tow. Said kid would skulk around behind the GameCube games while the ever obedient parent bought the game. On one occasion I remember quite clearing detailing certain scenarios of GTA 3 to a parent, only to be met with a blank stare and the words "Well, it's only a game innit".
Of course, the argument is that with a well planned campaign aimed at educating parents, they'll be less likely to buy their children whatever they want, but I think it's too easy to assume this will work. If kids are freely watching movies aimed at adults, a format which all generations understand, why will parents suddenly deem video games to be more harmful when they carry the same ratings?
You've also got to factor in how social gaming is. Half my gaming life was spent round a mate's house, playing the games he'd been bought that I didn't have, and playing together is becoming increasingly common. While little Jonnie's parents might restrict his gaming to 12-rated games and under, what about his mate big Dave? If he's got the latest GTA, you can be damn sure that Jonnie is going to be playing it with him. If a game is hyped enough kids will get hold of it somehow.
The Byron Review was absolutely needed. Its findings and suggestions are solid and few people can argue against them, but it's not going to suddenly put an end to kids getting hold of violent games. What it hopefully will do is make this industry less of an easy target for certain sections of the news media, with the fully legally enforceable ratings putting it on an equal footing to the movie industry. Which group gets blamed when the next game playing youth commits a crime is unclear, but you can guarantee that playgrounds and internet forums will be full of kids talking about their time playing GTA 4 come April 29. There's not a lot Byron or the government can do about it.