The funny thing I've found about the US Post is that the international rates, as well as being fairly inexpensive, are also pretty similar no matter what method you use. For example, if I sent a package by air (4-6 days or something like that) it's say $25. To send the same pkg by sea (6 weeks or so) is $22.
Where is the savings in sea freight then?
Also, my only advice would be to check HM Customs website...any posession of yours that you have owned more than 6 mos (?) you should not have to pay Customs on when you relocate...so check there and see if there is a specific form or something you need, otherwise customs can be pretty steep.