“… and you should be” or sometimes he says “… and you will be” what does the idiom mean? It seems like he’s being sarcastic, but what exactly, does he really mean?
I take it to be
…and you should be…like a steak…get it?..a well done steak!..ha ha ha
‘Done’ is code for a sped-up sexy party.
When “I was done” is used in writings by Dickens or Patrick O’Brian, the cockney who is speaking seems to be saying he was tricked, fooled, scammed or even beaten up.
You know, lots of people I’d mentioned it too seem to assume along the lines that Icarus: said – well done, like a steak, as in burned by the sun, the British stereotype of being pale, etc.
But if teela brown: is right, and he’s just quoting Dickens, and threatening to beat the wiseacre he’s talking to up, well, that makes more sense. Well, ignorance fought, thanks all.
Here’s an excerpt from The Pickwick Papers, in which Sam Weller shame-facedly admits to his father that he was skillfully scammed by another street sharp:
". . . ‘I got reg’larly done the other day.’
‘No!’ said his father.
‘I did,’ said the son; and he proceeded to relate, in as few words as possible, how he had fallen a ready dupe to the stratagems of Job Trotter."
It can also be interpreted as “ripped off” as in “ere he done me for 5 quid”.
Also for sex as in “I could do her”, or “I done her last week”. Working Class lingo mind you.
So its open to your own choice, but you knew with Benny he meant it “Dirty.” 
Just like Shakespeare! (I wonder how often that is said about Benny Hill.)
It means to have sex. ‘I’d do her.’
Hopefully the rest of the posters are just taking the mick by suggesting that anything Benny Hill said was about something other than sex.
Alternatively, I’ve see “done” used as “killed”, i.e. a Sherlock Holmes story titled “The Adventure of Charles Augustus Milverton” in which the title character cries “You’ve done me” after being mortally shot.
I’d take Hill’s use as a threat, not an offer.
If we could see a clip it should be easy enough to see what his intention was. It’s not like Benny was known for subtlety, bless him.
That would be a contraction of “you done me in”.
But here we are talking late 18th early 19th Century working class language.
People were not talking like that in the Benny Hill era, well not in every day conversation, unless you worked with the Monty Python Crew. 
As a British male of the era, I would take it to mean either done as in ripped off of done as in “fucked” most probably the latter.
Well, Hill himself was born in 1924 and influenced by music hall (if wikipedia can be trusted) so that some of his language (especially comedic catch phrases like the one in question) might seem old-fashioned can’t be that much of a shock.
We still do say ‘you done me in’ to mean ‘you killed me.’
But Benny Hill definitely means he’s going to do her as in shag her, and he’s going to do it well. Or somebody else is going to do her. I guess that could be seen as a threat, since being shagged by Benny Hill wouldn’t be most girl’s idea of a dream romance, but that’s not how he intends it.
“Done … and you have been”
“Done” means that the speaker has agreed a verbal contract with the other party.
“and you have been” means that the other party has “been done,” i.e. he has been conned.
Isn’t it also slang for getting busted, as in "done for(insert crime here)?
For all we know it could just be a TV friendly version of the classic exchange:
Indignant man: You should be caught and bloody well hung!!!
Comely lass: I think he’s already…
As I remember, he usually said this to the little old man that was a regular on the show.
In that case it would be done as in conned - context is needed. But not ‘beaten up’ - I don’t think suggesting that a little old man should be beaten up, or cooked, would go down well.
I’d forgotten that the Benny Hill show had anything in it but half-clad women and chases round parks with that music. Haven’t seen any of those chases for nearly 30 years I can still picture and hear them perfectly, but the rest of the show? Not a thing.
Nah, I remember Benny saying that on many a show (usually a bit different - “Well Done” (to the person, often the old man) “and you certainly should be” (usually looking at the camera with a smirk) - my young mind considered nothing other than the person should be cooked/burned up like a piece of meat.
Remember, this is the series which displayed the graffiti “Preserve Wildlife - Pickle a Squirrel today!” or the “Actress” saying “Look! In the Road! A Head!” and Benny as the director correcting her - “Look! In the road ahead!”