You can find the 13.4 in the USA from $265 ex tax, but you'll have to look for them.
Marv Golden has these headsets for around that price. He is based at MYF just outside San Diego and will take care of you if you have a problem (or not!).
The way the system works out in the US is that all prices are quoted PRE TAX! The price you see is not the price you pay, you have to add (usually) 8% on top. This is only payable if you purchase the product yourself in the shop or you live in the state where the shop is located (that's a bit of a simplification but for a non US resident it holds true).
So - you can either buy the headset in California, and pay 8%, or have it sent to you in another state (pay postage but no tax). You should work out the numbers yourself, and also remember it's nice to try them on first. You could always do that in a UK shop, decide which ones you like, and not buy them - but some might say that is acting in bad faith
I always buy this sort of stuff from "out of state" via a free phone number or web site. So if you are flying in FL, just buy from a CA shop. If you are flying in CA, buy from a FL shop.
BEagle is correct in that you should declare them on return to the UK and pay VAT, but those long haul flights confuse the heck out of me and I can never seem to find a customs officer when I want one
Clear as mud??