Reflectors first, won't get anywhere without decent reflectors. I'm not sure about the mirror film, you might get away with it as the beam pattern is produced through the glass rather than the reflector. I used kitchen foil and super glue for one of my indicators, works really well and is just as bright as the other perfect reflector with the lenses on. Wouldn't recommend it for headlights though, biggest challenge would be finding some decent glue that could stand the prolonged heat.
As for bulbs, the old type of fitment (with a round base) is called a P45 fitment, the newer 3 pronged fitment is called P43T.
I too have only seen Halfords sell a modern style Halogen bulb in P45 fitment but I'm fairly sure you have a few options online, from what I remember though none of them are the 'well known' brands.
You could also do what I did and get some
P45 to P43T adapters. These have worked well for me and I'm now using my trusty Halfords Super Brilliance +100% bulbs. Of the millions of bulbs I've had to replace in the Viva, these have lasted years of Mini and Viva use including a head on smash in the Mini, great light output too. With the adapters, I found I had to 'hollow out' the hole and prong slots a bit more so that the bulbs would fit well, and then cut the prongs down on the bulbs as they were fouling the headlight, but it's a 30 minute job with a dremel.
Or follow
this and put the P45 ring on the P43T bulb.
Make sure you put relays into the full beam and dipped beam circuits too, other wise you'll burn your switch out like I nearly did