Typhlosion: You have to get Cyndaquil as a starter pokemon then you have to evole it two times.
Dragonite: you have to get 2,100 coins by playing voltorb flip in Goldenrod City Game corner and then buy a Dratini. It will evole at level 55.
Dunsparce: it can only be found be chance if it appears when you use rock smash on a boulder.
Slaking: after you beat the johto league and get the national Dex, you have to headbutt trees on Route 25 until you get a Slakoth. it evolves to Slaking at level 36.
Mewtwo: When you can go in Kanto Newtwo is in Cerulean Cave. But save before you try to catch it because it only comes 1 time.
Arceus: You can't get him in UK yet.so you either have to trade with someone who already has Arceus or wait for the UK Arcues mystery gift event in 2010.
Steelix: Steelix is in Route 47 and Mt. silver. but if your friend has Onix, they can make it hold metal coat and trade it with you then it'll evole to steelix.
Nidoking: Catch a male nidorann on Route 35 or 36 then it evoles to Nidorino at Lev. 16 then use the moon stone on it to evole it to Nidoking.
Metagross: when you beat Red at the end or the game, you have to go to Steve Stone at Pewter Museum of science, Give him a forretress and then you'll get a beldum, then evole it at lev. 45.
Gengar: first catch a Gastly in tin tower or sprout tower, then evole it to Haunter at lev. 25 then trade it with someone and it will evole (but remember to get it back).