Regular posters should realise that my policy is "innocent until proven otherwise". Betfair have a history of this kind of thing, so whilst this does not prove they were necessarily in the wrong here, it is further evidence that, on balance, that Betfair are using an arbitrary "management decision" in this case, rather than a breach of the terms. Further information needs to come from Betfair, but they no longer cooperate with CM, so mediation is out. Although the ASA can't do anything about getting the player paid, they CAN steadily turn the screw on the company to start "playing fair" under both the wording, and "the spirit" of UK consumer law. Complaining to the ASA ensures this issue is "on the record" as far as UK regulatory bodies are concerned, and this carries more weight than listings in internet "rogue pits", however well respected the website.
The UK government are already aware that this is a more general problem with sites manipulating their licensing arrangements so that they can have full access to the UK market, yet dodge having to fully obey UK consumer protection regulations by forcing players to take legal action in a foreign jurisdiction, or complain to a foreign regulatory authority.
If the OP is a fraud, it will eventually come out, not that it will elicit too much sympathy for Betfair
It does not seem that bonus rules, even "spirit of" can apply as the OP bust out from the bonus, and won from the comp points. Betfair awarded those comps as part of the program, so it would be the rules of the comps program, not the bonus terms, that would apply.
The next step would be to look at the OP's actions against these rules, and see if there is any breach there that can lead to the voiding of winnings.
The OP didn't get any explanation from Betfair as to what rules were broken, his account was closed and all his money confiscated, including his DEPOSIT by the looks of it. If they are carrying over the bonus rules to the play with the comps, they should also be carrying the DEPOSIT along with it that initiated these rules, and at the very least refunding it, as any offence related to bonus play would have taken place during the play with the bonus. The OP could have deposited again, rather than use the comps, and Betfair seemed to be happy taking further money from the OP whilst he was losing because his "offence" went unpunished.
One thing tha ASA can do is require Betfair to no longer have misleading terms on their website, and whatever rule they used to confiscate the OP's money would have to be added to the terms of use. If it was simply "whim of management", this will cause problems for Betfair, as they would have to put it into words, and apply it to all players to show that things are "administered fairly".
Another useful avenue would be Trading Standards, who have a different set of powers at their disposal. They will often negotiate on the complainant's behalf, which can act as a strong laxative for corporate constipation (my own personal experience
).
Asking a local MP to look into the matter could also be useful, after all, it is MPs that will be debating and implementing any changes to the UK licensing arrangements for offshore casinos and sportsbooks.