It's probably worth prefixing all this with the caveat that, as discovered from the discussion on your previous question, this does vary quite a bit between from place to place (and possibly between classes, professions, sexes etc...), so do beware that what answers you get may well not apply to the whole of the UK :)
I'm not sure which films you've been watching, but the reaction you've observed may have more to do with the tone the words are used with: if you walk into a pub and call someone you don't know a cunt, you can be fairly certain of a fight. If you instead call them a wanker, your chances of getting away without a fight are better, but only slightly so. With either word, anyone being referred to with it is likely to get very angry.
That said, just saying the word wanker is much less shocking than saying cunt: in the place I work (which is admittedly rather tolerant of so-called "bad language"), people might often use the word to express their frustration with someone (though never anyone working there) — say, a persistent and rude telemarketer who won't leave them alone, or a particularly unhelpful customer service rep from the phone company. Or perhaps someone who cut them up on their drive to work. Use of the "c-word" is much more rare (that would be reserved for someone who had, say, driven into their car and written it off...). And it is telling that there is no corresponding expression "the w-word". (Well actually there are many possibilities — 1, 2, 3, 4, 5 — but none of them refer to "wanker"!)
Of course the dynamics of insults are very different between close friends, where much stronger language can be used, and indeed being incredibly rude can often be a mark of affection. Tone is everything, and I would never advise doing this unless you are totally sure what you are doing!
Do means something along the lines of party in this instance. A post-wedding party, on the evening of the wedding, probably.
OED has:
DO, n.1
b. Something done in a set or formal manner; a performance; esp. an entertainment or show; a party; hence (orig. jocular), a military engagement, raid, or other ‘show’. Orig. dial. or vulgar.
and some examples of its use first attested to from 1824:
a 1824 J. BRIGGS Remains (1825) 243 Such individuals should have their feast (or do, as it is called).
1925 FRASER & GIBBONS Soldier & Sailor Words 78 Do, an event. A stunt. An attack, etc. E.g., ‘When is the do coming off?’; ‘The Somme do’; ‘The Havrincourt do’, etc.
1955 Times 18 May 14/2 Miss Margaret Herbison broadcast on behalf of the Labour Party last night a talk which she described as a ‘family do’.
1958 M. KERR People of Ship St. ix. 108 Her family has a ‘do’ every year on the anniversary of the day her mother's father died. Ibid., Christmas ‘dos’ are especially important.
Best Answer
It's Bostonian. The concept comes from the thalidomide babies born with "flippers" for limbs in the late 50s. The idea is that instead of arms or legs it was your head and your brain that were stunted and came out as a flipper. It's a way to say you're a monumental, irretrievable, freakin' idiot.