From someone who moved to Japan to continue his study of martial arts...
If you're interested in self-defence then martial arts are a bad investment. It takes a lot of time and money to get good, it's very hard to train with enough realism for it to be effective, and no matter how good you get, luck, the element of surprise, weapons and so on can override years of training experience. Self-defence is much better served by learning when to leave a situation, how to de-escalate, not putting yourself in unnecessary danger, and generally looking after yourself.
But the original question is an interesting one so I'll do my best to answer it. Martial arts are designed with a purpose and philosophy in mind. Krav Maga is for military combat, boxing for sport, aikido for spreading 'harmony' (和 / wa), jodo for defeating a sword with a 4-foot stick, etc. Basically none of them were designed for 'street fighting'. Most of them are designed for either killing, or for sport, and there's not a lot in-between.
There's a reason why martial arts aren't designed for the 'street' though. From the perspective of old school martial arts, violence is a serious business and you're not playing around. There's no ego, no friendly scuffle, no posturing, no "teaching someone a lesson", no half-measures of any kind - if you use it, someone is getting seriously injured. But the world is a lot less violent than it used to be, so martial arts had to adapt to keep their relevancy and many of them became combat sports. All the lethal techniques are thrown away, as well many other self-defence considerations such as one-on-many drills, weapons, realistic clothing, 'fighting dirty' tactics like biting, scratching, concealed pre-emptive strikes, etc., etc. So that leaves an aspiring street fighter in a difficult position.
In modern society you're bound by law not to use excessive force, so you have to ask yourself, what do you want? You could learn a Filipino martial art using a butterfly knife, be an amazing street fighter but the first time you defend yourself you go to jail for over a decade for knife crime. Or you could learn a sport based martial art and try to mash it into real world situations, and as soon as you're in a confined space, sitting down, someone grabs a pool cue, or anything you didn't cover in your sport, everything goes out the window.
Street fighting requires a mix of realistic elements plus restrained elements (assuming you don't want to kill or permanently injure), so if the question is which martial art is the best for the streets of Britain, taken alone they're basically all terrible because it's not what they were designed for. You have to build it yourself from your own experience, which is typically what street fighters do. They mix everything together, gain experience through real world scrap ups, and end up with completely unpredictable, wild fighting styles.
But if you don't want to become a street fighter this kind of question is totally academic - unless you're in a gang or something, street fights are largely avoidable. If it's not academic, then I can't say I recommend it, because even putting the legal and ethical issues aside, I've met some of these guys and their bodies are messed up. Missing teeth, chronic pains - street fighting gets you injured.
Besides, the real benefits of martial arts are discipline and becoming a better person. This is kind of wasted if you use it to become a street fighter.