It is a pretty universal rule on all airlines that when fare bases are mixed on a single ticket, the most restrictive conditions apply to the whole ticket and this is what presumably happens here. Looking at the fare conditions now will probably only confirm this and will not be particularly enlightening.
The only issue is what was actually displayed when the OP made his booking. If all that was displayed was what the OP states, then the OP has good cause to complain and BA could not oppose fare conditions which are contrary to what was displayed. It may be, however, that the OP only saw part of the rules and missed the bit about combination of fares. The OP may wish to do a dummy booking and look carefully at what was displayed before he pressed the purchase button.