its the most complete list i know of for UK firms, theres a few directories of US firms elsewhere but havent found anything useful to UK that comes close to it.
Seatra was set up by someone I have worked with in his capacity as both a trader and a risk manager at a UK based arcade. This person has been involved with the markets for over 40 years and wanted a change of lifestyle. The problems he encountered mainly centred around recruitment and connectivity to exchanges (simply because he was running a small remote outfit, so they were reluctant to send people out to solve the issues). As a result of these factors, the firm ceased trading and the person in question now works in Gibraltar as risk manager for one of the arcades.
Arcades are exactly what they are cut out to be. That doesn't mean they suit everyone, but having worked at one for over 8 years I can only suggest that the things you learn from people like the guy in question, are worth their weight in gold.
Also, Mac Internationals was/is a division of MacFutures in Gibraltar. When Refco took over Mac and then went bust, the International division splintered several ways under different names. The majority of traders from this company are still trading either with the re-branded entity or at another of the arcades.