I think there haven't been too many replies because this has been discussed on an earlier thread that started around launch time, a thread that I am too lazy to reference :-) I'm in the US, but I included Amazon UK links for you, because that's how helpful I am.
I'll put my two cents in here. My current 3DS accessory setup starts with a
HORI clear case for my 3DS. They also sell a silicon case, which I haven't tried, but I'm willing to bet it's good. HORI is a trusted brand for me, they always make very high quality accessories, miles better than their cheaper competitors like PDP. On my screens I've also got
the HORI screen protectors, which are so awesome. The top screen protector protects the entire top bezel and not just the screen, which is important because on the 3DS it is flush, flat, and shiny.
I also carry around my 3DS inside the
HORI hardshell carrying case, which has room for 3 games as well as a pocket for microfiber cloth and AR cards. It's not a perfectly snug fit if your 3DS is "naked." But if you've got a silicon case or clear case around your 3DS, the hardshell fits pretty perfectly.
And just to prove I'm not a shill for HORI, I also carry around the
Gear Monkey DS/DSI hip pouch, because I want the world to know I'm a complete spazz. Just kidding. But, what I like most about this, more than how well it stays on my belt and how secure it is, is the fact that it is non-descript. It is the king of non-descript. Everyone assumes that I've got my smartphone or something in it, and not my 3DS. In fact, there's nothing about it that would lead someone to know that I'm a complete loser who carries around my 3DS ALL THE TIME. Now, there's a little pocket on the front where the manufacturer actually thinks I'll keep an extra game cart, which is ludicrous, so you can go ahead and ignore that. I like to keep my games and not lose them, thank you very much. If you really want to purchase one of these, good luck because they're getting harder to find. I bought mine at a Toys R US bin sale for $2.50 US. It's not available on Amazon right now, but maybe you might have some luck elsewhere.
That is the secret to my streetpass success. 90 Mii's and counting!