I've shipped basses to England and used USPS without issues. their tracking system is a bit of an under performer once your package arrives at customs (my last shipment went without a tracking update for several days, and then suddenly the local carrier called my client noting the bass needed to pay VAT immediately)
I just did a cost estimate from Seattle to England for a 22 pound 8 ounce package measuring 8"x20"x52" (my standard shipping box size) and I get a shipping estimate of 6-10 business days in transit for a bit under $95US (about $90US if I pay on-line and drop-off at the PO) For reference, here's my search:
http://ircalc.usps.gov/MailServices.asp ... g=0&r=True IIRC shipping FedEx, DHL, or USPS would have cost somewhere around $350US. the advantage with this higher cost service is that you have better protection for damage and loss, and the tracking is significantly superior. getting a feel for who you're selling the bass to should help you decide if you need the extra protection at the higher cost
all the best,
R