Hi GOM,
This link might be part of the explanation for differing speed test results, as might other unknown simultaneous internet traffic or possibly even contention.
http://www.thinkbroadband.com/news/3902-not-all-speedtesters-up-to-the-job-still.htmlProvided you are confident that you have used the test socket on the master socket, or temporarily isolated any star and / or extension wiring, the BT O threat is almost never carried out.
The BT O guys are usually far too busy to want to become involved in such arguments and most are quite genuinely trying to get the best possible connections for their clients from the creaking copper within their allotted 2 hour time slot. I hope you're pleased that your case isn't just being dropped as the line test has passed.
Regarding other ISPs, I expect you will find some have far better fault processing staff and facilities than others as has been reported many times here.
Apart from the sheer frustration, discussing a problem in plain english with a trained troubleshooter, rather than to an automaton reading from a crib sheet, is a most welcome benefit in otherwise quite stressful circumstances.
Kind regards,
Walter