I can't find anything on the website to answer the questions I would have, but as the company doesn't bill separately, they must get their revenue via a proportion of the call charge made by BT (or whatever TelCo you are with). My guess would be that they issue a "special" number, possibly something in the personal numbering codes (07xx).
Frankly, with all the deals around on long-distance/overseas calling plans (e.g. the 1899 mentioned above), it seems unnecessarily restrictive. Why go to the trouble of having to assign a separate number for each U.S. line you might want to call when you can just get a cheap calling plan and dial anywhere in the States using the regular NANP number?
As for dialing the U.K. number with an alternate carrier, be aware that rates for calls to non-geographic numbers (including 07xx and similar) can often be considerably higher than for a regular long-distance call. Some carriers won't even allow calls to certain codes.