Cheapest courier

Try delivering directly to cargo airlines so they can be claimed in the destination airport.

Can you elucidate please? 

United airways have best cargo rates for shipping to UK
find out http://www.unitedcargo.com/shipping/productsAndShipping.jsp?name=Quickpak&type=product to provide precise information about your package to get rate.
You should save near half of what you've been quoted by major carriers.

If you know someone with a DHL account, the rates are a considerable fraction of those quoted as "retail." I don’t really do much shipping, but a lot of buying, so it doesn’t help me too much, but I did get a quote from them at one point, based on a minimum assumption of volume. When I’ve shipped stuff back and forth to Slovenia (Kuzma), I used DHL one way, Franc used TNT, i think, based in EU- I’d get an email mid-day and the courier would be at my door the next day before noon in NY. Crazy!
The key factor, aside from carrier is obviously packing/crating. And the 3d party crating services aren’t always "experts"- based on my limited experience. Good luck!

Thanks czarivey & whart...

CZ,

Actually USPS will be the cheapest, if you ship directly with an Airline, which will be exceedingly difficult if your not a "known shipper". Since this is an international shipment OP will have to clear customs for the speakers once they reach the UK with all the fees, bonds and paper work that involves. Then there is local transport from the airport to consignee etc. For a one of shipment USPS /FedEx/UPS/DHL certainly would be the preferred choice, since all the above is included in their price.

Best of Luck

Peter