The most economical way is first class assuming the package is under 64 ounces. If it's over, it has to go priority. About a year ago or so, surface mail (or sea mail, as it used to be called) was eliminated. However, if you plan to insure it, it has to go priority, which is more expensive.
I've never had a problem with a lost package to the UK or anywhere for that matter, but if it means a lot to you, you could insure it. However, let's look at the practical side. If it gets lost, you'll have to deal with the post offices and good luck, from what dealers have told me. Recently, I sold some Trophy sets to a buyer in the UK and the buyer claimed he didn't receive them. I asked around about filing a claim and everybody told me you're better off just refunding the money. Fortunately, it had gotten stuck in customs and the buyer received it shortly thereafter.
If you have any other questions, please let me know.