Well my stockbroker certainly allowed it,
although I must admit the guidance from the inland revenue is not crystal clear. In their recent "Guidance notes for PEPs and ISA managers" in section 7.74 they list all the recognised stock exchanges around the world - and it is a big list. The bit that pertains to OTC is
"Any stock exchange in the following countries which is a stock exchange within the meaning of the law of the particular country relating to stock exchanges (or as specified below)
Austria, Belgium.............
USA - any exchange registered with the securities and exchange commission of the United States as a national securities exchange
USA - the Nasdaq stock market as maintained through the facilities of the National Association of Securities Dealers, Inc and its subsidiaries"
Well I would say it is a stock exchange (Is that an exclusive "or"?) and it must be registered with the securities and exchange commission, although I'm not altogether certain it counts as a national securities exchange (although the exchange certainly operates at a national level).
Views from those with greater expertise appreciated