For public domain use:
Here is some information I dug up for an english essay at school:
Rumors and faq's:
1. Posts on usenet/irc/message boards are public domain.
FALSE. Posts are (assuming they are not copied or general knowledge) are original works of authorship, and are protected by copyright laws, unless it specifically says it is for public domain use. E-mails also fall under this category.
2. Copyrights must be registered to be effective.
FALSE. Copyrights are effective as soon as the work is placed in a fixed format. Registration is optional, and can lead to greater monetary awards in court.
3. If it doesn't say otherwise, it's public domain.
FALSE. Under a convention (I don't remember the name) on March 1st, 1989, the law was changed so you don't have to post a notice for the copyright to be effective.
4. A name, title, word, slogan, or idea cannot be copyrighted.
TRUE. These do not fall under government restrictions for copyrightable material.
That's all I can think of right now, I'll post more later.

Good to know, since copywright issues are causing lots of problems on the net right now. I for one think these laws need to be changed, but that is just me.

If I write something and write © www.musichere.co.uk does that mean I own the copyright, even though in case someone else uses it and claims it to be theirs, that I can do absolutely nothing about it (?). Could I threaten legal action though? Even though I don't have a registered copyright (which costs $40)?

In the US, yes, as all things are copywriten as soon as they are created here, whether or not you register them. Registering them just allows for increased legal protection. I have no idea how similar the UK is to the US in this regard though.

Problem with the web is that there are users from a lot of different countries, and countries copyright laws differ quite a much.

An example, that I read from a computer magazine recently, sprung to my mind. In Russia there is a certain electronic music store (I do not wish to advertise them here...) that, because of the lot looser copyright laws of Russia, can offer material that is not available in western countries. And this is perfectly legal as they are operating in Russia and follow Russia's law.
The magazine mentions that they actualyl have music by the Beatles available to download for payment. In western countries yet no music by the Beatles has been released to be sold electronically.

It actually depends if www.musichere.co.uk is a physical entity or not. Most copyrights are owned by an individual, such as my works of photography. Since I myself created them, I myself own the copy right. However, what you ask is if www.musichere.co.uk owns the copyright. This can only be true if www.musichere.co.uk is the legal name of a corporation (that also has a registered ficticious name statement already filed). In that case, the corporation owns the copyright, not the individual. You have to be careful with this sort of thing, as if you sell the company, you lose control over the copyrights as well. Hence, in my old photography business, I copyrighted everything under my name, not the business name.

People are right when they say that registering with the govt. only basically gives you an added monitary award if you sue someone for infringement. However, it also establishes that you created it first. Suppose you post something on a website, and then a week later you find it somewhere else. You try to sue, but you'd have a hard time seeing as the other person could easily say "I created it first, so it's therefore mine." What do you have to counter that? If it's a work of art, you could show the original file, but if you wrote something (poetry, essay, other), it's basically your word against his. Thus, registering it solves that problem.

And about registering names and whatnot, of course you can't do that. That's what trademarks are for.