Red lasers wont be a beam, just a dot. Unless you are in smoke, fog etc.
Green lasers are usually produce a visible beam however are usually more than 5mW.
UK legal ones are maximum at 5mW. You technically can import more powerful ones legally, however most reputable sites will not allow anything more powerful than 5mW.
Go for a 5mW, they're not really that powerful but you can use them at most sites.
Range wise you will get about 30m before the dot becomes quite faint during the daytime outside. Indoors you generally dont need to worry.