As far as I can figure the UK is much like the US or Canada in respect to carrying lockpicks, but as far as mailing them I can't find any laws that say you have to be a locksmith or that you have to verify you are shipping to a locksmith in the UK. Different cities, provinces, countries the laws may change, but in general I can't find a law that says so. Someone in who lives in the UK might have a better idea. As for what to identify the package as to customs... tools, handtools, hobbiests tool set, is discriptive enough. It isn't deceptive, like doll golf clubs or swizzel sticks, but it isn't specific either. Even though it may not be illegal to ship picks wherever you are shipping them I can pretty much guarantee "lock picks" is going to gain the attention of whoever handles the package. If you absolutely need to identify the specific contents go with hobbiests lock tools. Try to steer away from locksmithing tools if you aren't shipping to a locksmith. The negative stigma associated with the tools is not going to help your package get to it's destination any faster, and so I would dance around using those words exactly. Just sweat profusely and say, "well it isn't a bomb... if that's what you think" and you should be fine with just labeling them "burglary tools".
If you do ship them to a country where it is illegal to own them, Poland for example, then "tools" is not going to get you off the hook if they find the contents to be illegal. Technically a knife is a tool, but it would still be illegal to mail a bowie knife to New York under that content heading. Ignorance is no excuse for breaking the law, blah, blah, blah. Do your best to find out the laws of the exact places you are mailing to beforehand, so you are comfortable in the fact you aren't doing anything maliciously illegal. I don't think you have anything to worry about, but I'm not a practicing lawyer in the UK. All these laws are so much grey area it's difficult for the hobbiest to sort it all out... if you feel apprehensive about mailing, just don't until you find out for sure. Sorry I couldn't be more help.
-Tooly