Both have phone numbers and Email addresses. You can ask them if they make the subwoofer amplifier for Parts Express if you wish. The pictures at Keiga do not seem to match the pictures for the Parts Express amp, but who knows? The Keiga products look interesting anyway.
I would be surprised indeed if Keiga or Yung would sell you any of their amplifiers, but they very possibly can supply you with a list of retailers and/or mail order houses that will. In my experience, manufacturers have no qualms at all letting people who know who carries their product. It is the retailers who don't want you to know who the manufacturer is.
If there are no retailers in England, you can ask about other EU nations. Isn't there some kind of low tariff arrangement there? For that matter, maybe there is some Taiwanese mail order house that can handle it-I don't know the tariff situation between England and it's former colonies. Worth asking.
If nothing else, you can have fun trying to chase this subwoofer amplifier down, or locating a suitable substitute.
I have no doubt Thingyness is absolutely correct about Proton. I have heard of that company before. A salesman from a retail chain of better quality audio/video cited one of their products as the best thing they sell. Anyway, here is the USA page, which gives references and contacts to the home office. I would try the Audio Products division first. http://www.proton-usa.com/sales/other.htm