First Class is the best way to go I've found, downside is you have to source your own packaging but it's cheaper. Also, for the UK first class will be picked up by Royal Mail who will take it to your house. Priority Mail will be picked up by Parcelforce who will take it to customs so they can add their own clearance charge because they are thieves.Honestly, I have yet to find a cheaper option for international shipping than USPS First Class. It's about double what a domestic shipment would cost, but its going to another country, so it seems like a legit trade off. Certainly still less expensive than any other method I've looked at.
I always use Royal Mail first class for everything I've sent to the States and so far everything has got there okay...latest couple are taking their time though.