This may be a similar problem that I have had in other countries. They don't allow their own currency to leave the country, and they require dollars to exchange for local currency after you have arrived, thus boosting their currency reserves. The conflicting advice may be because the US Dollar is now much less popular than it was. Travelex do not quote a rate for Cuban pesos. Cuba would be very unusual if dollars were not accepted, but they may well accept British money just as easily.