Well, any depiction of Mohammad is considered to be blasphemous from an Islamic perspective. It's not hard to see why an "ordinary British Muslim" who seemingly takes his religion seriously would find the cartoons unacceptable. The important thing to note is that they all condemned the attacks. If they went around showing support for what happened, then that would be an issue.
Well, look around and see how many people mock Christianity in blasphemous ways. And even though some people are offended by it, the world still keeps spinning on like nothing has happened. Look at Madonna. She is still alive, and still criticising the church for over 30 years. Christ is often portrait in blasphemous (if you want to use that world) frequently. I have plenty of practising Christian/Catholic friends and if, for example, I post a image like
this on Facebook (which I actually did), people will either not [Like] it, or if they are really really offended they will unfriend me. I don't have to fear if extremists will arson my house or if I will be violently targeted later on. None of my very, very, VERY Catholics aunts stopped talking to me because I posted that. But enough with silly examples and comparisons. My point is:
In the end, these are all religions.
Religions you are not obliged to comply with, if you don't believe in them. Mainly because if you had to comply to every rule of every religion to not offend anyone it probably wouldn't be possible to live. We live in a secular state (speaking for myself) and I would like if it would remain secular. I agree there is little to no point in posting some of the images of the prophet that have been posted (and my example might be too mild compared to a few of those drawings), but then again, they are trying to de-sensitise it. I respect everyone's right to practice their faith, but they have to respect my right to not practice it as well. But it is extremely scary that some people today, with access to information and growing up in developed countries like the UK, still think that Charlie Hebdo brought it upon themselves, and even scarier to think that some people would actually say that they deserved it, because you have more empathy for your imaginary friend than for other (human) people.