"Hit/clout/slap round the head" is a UK idiom. It doesn't imply once or more than once.
Examples from the BNC:
"clouted it very hard round the head with his stick"; "clouting him round the head"; "getting a tap round the head"; "who beats you round the head"; "Because you can't wallop them round the head can you?" and so on.
There's a more specific form "round the ear": this is nearly always "clip round the ear" (17 times in the BNC) but also "smack", "clout" and "belt" (6 instances in total). ("Clip" meaning a blow only occurs in this one phrase, as far as I know, but as a verb it can be used to mean "make an accidental, glancing contact", as "I clipped it with my mirror as I went past".)
Quite why we say "round" is not clear: it is certainly an aphetic form of the preposition "around". It doesn't seem to occur with other body parts than heads and ears (there are a few possible instances in the BNC, but they all might be interpreted as a literal "around"). It doesn't seem to imply any particular part of the head, or multiple blows "all round the head", as one might have supposed.
Asylum refers to political protection. It is given to those who are persecuted for something in one country, and wish, no, are in need of sanctuary in a country that will not imprison, torture or otherwise perpetrate human rights violations against the individual who is at risk. Granting asylum is a measure that transcends and supersedes all international laws of immigration that might otherwise be in place.
Exile can be involuntary OR voluntary. Involuntary scenario: Emperor Napoleon (the first one, I think) was exiled by the French government to the Island of Elba.
All instances of exile are not punitive though. Here's an example of going into exile in order to avoid prosecution (not persecution), motion picture director Roman Polanski. Polanski chose exile to avoid criminal charges in the United States.
Voluntary scenario: Hypothetically, Polanski might have chosen to leave the U.S., in a self-imposed exile, to get away from the emotional anguish he associated with the U.S.A, where his 8+ months pregnant wife, Sharon Tate, was murdered by Charles Manson et. al. I mentioned that just for illustration purposes. It is well established that Polanski was a fugitive from justice and went into exile to a country that didn't have mutual extradition laws in place with the U.S.A. (And, unlike Gaddafi, Polanski was not accused of a crime as serious as mass homicide, which is a reason the country he fled to was willing to tolerate him.
Corrupt head of state example
Baby Doc Duvalier went into exile in France, once his oppressive totalitarian regime was overthrown by the people of Haiti. It was not so much a matter of France offering him political asylum, as France tolerating him to live there, an ocean away from Haiti. That's an example of exile, rather than asylum. It is involuntary, as Duvalier had to get out of Haiti. It is voluntary, in that he chose France (I guess).
Regarding @Mike's follow-up comment,
So if asylum is the act of asking for protection and exile an act of being sent away, why are we talking about "corrupt head of states going to exile in another country"... it should be "head of state asking for asylum" or "country offers asylum". Why is that?
@Mike cites a specific example, a Reuters news story about Gaddafi. However, this was NOT a situation where another country offered asylum to Gaddafi (the corrupt head of state that Mike referenced). Here is the relevant part of the Reuters article:
Burkinabe Foreign Minister Yipene Djibril Bassolet said that Gaddafi
could go into exile in his country even though it is a signatory of
the International Criminal Court, which has charged him crimes against
humanity.
"In the name of peace, I think we will take, with our partners in the
international community, whatever steps are necessary," Bassolet said,
without giving any other details.
It is important to note that Burkinabe Foreign Minister Bassolet said Gaddafi could go into exile in his country. He was willing to tolerate Gaddafi as an exile in his country only for the sake of expedience, to end civil war in Libya with associated loss of life. It was offered for the greater good of peace.
That is very different than granting political asylum to Gaddafi! It offers no guarantees of permanence unlike being granted political asylum. Being allowed entry as an exile, under conditions of duress (e.g. as Burkino Faso proposed), is an emergency measure. Gaddafi would have had to accept whatever terms he was offered by Burkino Faso, even it meant house arrest or confinement in primitive circumstances in exchange for assurance of his physical safety. The world community would have censured a country who was a signatory to the International Criminal Court, yet gave Gaddafi asylum.
This is the distinction between exile and asylum, although you may notice the words being misused, for political reasons, at times.
Best Answer
My English teacher at school was adamant that on to was always two words, a position which is acknowledged by ODO:
In British English, it’s always safe to separate on to; in American English, onto would appear to be acceptable in almost all circumstances.
If onto can be one word in the examples given, one has to decide whether on can be classed as an adverb with to as the preposition, or whether onto is an acceptable preposition in its own right.
In Burchfield’s New Fowler’s Modern English Usage he quotes Fowler from 1926:
With hold on, the on has Fowler’s “independent status”, and there is no sense of movement as there is with driving “onto a beach”. Indeed, the act of holding on to something is precisely to steady oneself and prevent movement! Hold on to is therefore appropriate.
In the second example [“grabbed onto the cushion”], with the verb grab on the to is needed (so on is not independent), and there may even be a sense of movement with a sudden taking-hold of the cushion. Grab onto might therefore be appropriate.
That said, customary forms in different dialects of English may dictate a different use. Grab onto looks decidedly wrong to my British eyes, conditioned as they are, even though I’ve argued fairly successfully from Fowler [a respected British authority] that it’s reasonable; and hold on to may look decidedly wrong to American eyes.