Before decimalization of the British currency in 1971, any amount of money ending in -pence was pronounced /… p(ə)ns/: twopence (usually spelt tuppence) (/ˈtʌp(ə)ns/), threepence (/ˈθrɛp(ə)ns, ˈθrıp(ə)ns, ˈθrʌp(ə)ns/, fourpence (/ˈfɔːp(ə)ns/) etc. The adjectival forms ending in -penny were also reduced, to /p(ə)nı/, with the same reductions of the numeral, including halfpenny, which became /ˈheıpnı/ (final /f/ elided) (the pronunciations of tuppence and threepence show just how longstanding these forms were: tuppence must represent the shortening of /uː/ to /u/ and predate its split into /ʊ/ and /ʌ/ – spelt tuppens, the OED has an example from 1514).
Once we decimalized, it became necessary to distinguish between the old penny and the new (the coin worth 6d in ‘old’ money was retained for some years, but was now worth 2½p in the new). The way we all did this, I suspect without thinking much about it, was to use the strong form for the new value. I had half expected that when the new currency became familiar, we would all go back to our old ways, and the weak form would re-emerge, but this has not happened. I tested it in the early 1980s by asking for some 10½p stamps at a post office, and pronouncing the value as /ˈtɛmpnsˈheıpnı/. The counter clerk – older than me, and so well able to remember pre-decimal money – asked me to repeat my request, and it was only when I eventually said /ˈtɛn ənd ə hɑːf ˈpɛns/ that I got the stamps.
I started with two weak forms lost, but I think that makes three: unless someone can correct me, I think that half was only ever weakened when followed by -penny or -pence as an amount of money (i.e. not the family name Halfpenny), so that it is no longer heard.