The letter you refer to on the Ford RS Owners Club thread is here.

There are also some links you may find useful to Advertising Standards Authority (ASA) and others here.

As for the title about clarification on the law, I'm not sure what you're asking. There is no law that says any organisation must put you through when you ring its switchboard or another department. They can refuse to give you an alternative number.

If others have said that they have called the switchboard and been put through, and they won't now, maybe they have changed their policy because that many pesky supporters keep ringing.

Surely with such a 'distinct' fan-base, and presumably forums etc for Man United fans, a campaign against this rip-off of loyal fans could easily be started - by the fans themselves.
Perhaps many of them don't even realise the 0870 scam.  First steps - spread the word, organise petitions, mobilise your people!!
I wouldn't be too confident however, when you see the 'fans' paying extortionate amounts to watch games, then spend ridiculous amounts in the United shop for shirts etc. - I believe the cost of 0870 phone calls doesn't even register on the same scale, for them.
Remember football used to be a game - now it's big business, so they are looking to empty your pockets at every conceivable opportunity, and call you 'Loyal Fans' (as opposed to 'Loyal Mugs' ) at the same time!!.  Brilliant marketing.