pronunciation of "conservative" and "expensiv

I was recently listening to a cassette which accompanies a British ESL programme called "Linguarama". There was a conversation between two women (a customer and a sales clerk). The woman was buying a shirt for her husband and described him as being conservative. When told that the shirts were 4.50 pounds each (this is a cassette is from 1978) she remarked that they were too expensive. As I was listening to the conversation I could have sworn I heard the woman (who is supposed to be an RP speaker - actually ALL the speakers on these cassettes are supposed to be RP speakers for learning purposes even though many sales clerks do not use RP) - anyway, I could have sworn I heard her pronounce the word "conservative" with an 'f' sound at the end. I figured I must be hearing things so I rewound the cassette and this time I paid closer attention. It turns out she DID use an 'f' sound. She did the same when she later pronounced "expensive". Thus the words were pronounced, "conservatif" and "expensif". Now I've watched enough Hollywood movies, American TV shows, and listened to enough American politicians to know that these two words have a 'v' sound in General American. However, I am also extremely familiar with RP. I watch the BBC World News, I listen to British actors, and I've even read a good part of the "Accents of English" book by JC Wells, especially the part where RP and GenAm are compared. I was never aware that RP uses the 'f' sound in the endings of such words as those mentioned above. Now, it seems to me that there are five possible explanations for this phenomenon:

1) I have a hearing defect in which v's sometimes sound like f's (unlikely).
2) The speaker on the tape has some kind of speech defect in which some v's are pronounced like f's (also unlikely).
3) RP does indeed use an 'f' sound in the ending of these words but it's just something that has eluded me thus far simply because I haven't been paying close enough attention to the BBC announcers and to the British actors or else I haven't been reading the JC Wells book carefully enough.
4) Only some RP speakers use the 'f' sound while others do not (they use the 'v' sound) depending on individual taste and idiosincracy.
5) The 'f' sound at the end of such words is typical only of upper-RP and falls outside the domain of mainstream RP.

Can someone please tell me which of the explanations above (if any at all) is correct? Any other information would be greatly appreciated. Thank you.

All I can say is that I have a slight tendency to partially devoice these consonants, especially if they occur before other words that start with voiceless consonants. I wouldn't say that it actually comes out as an 'f' though (which is identical to a completely devoiced 'v'). The voicing is very weak in "expensive car," for example, although it's still there. I speak GAE, not RP, however, so I'm not sure what RP speakers normally do.

I'll have to ask some people I know who actually speak RP to pronounce these words and see how they sound.

I think you're probably right about her sounding an "f" instead of a "v", but you are probably scrutinizing the pronunciation to a greater degree than would be expected. "ive" at the end of a word should always be pronounced as such, but no-one is 100% accurate in their pronunciation so in her own personal pronunciation she may have sounded it more as an "f" sound, which most people would not notice.

Also, since English tends to have long vowels before voiced sounds, partial devoicing of voiced sounds may occur with no loss in comprehension because speakers are aided by the long vowel preceding it. In fact, it's pretty common for English speakers (no matter the dialect) to partially devoice final devoiced sounds because the preceding long vowel helps maintain the distinction. For instance, compare the vowel in "bit" and "bid." The one in "bid" is typically longer.