Fit as a butcher’s dog

Had not heard of that phrase until now
will definitely use it for its latter aforementioned meaning
:0)

Although (being not from any part of England) I have never heard the expression, it seems fairly straightforward and apt. Incidentally, I believe that the expression would be called a "simile," rather than an "idiom." The latter being an expression that seems odd if interpreted literally, unless you are already familiar with its meaning, such as "touching base" or "bending over backwards."

@SinTax'ed Enough - I take your point about it being a simile, but I wonder if you'd have found it quite as straightforward if you hadn't read Graeme's comment first; it certainly didn't hit me that way when I first heard it. My first thought was that a butcher's dog is as likely to be overfed as particularly fit.

It is also peculiar to one part of England, and unknown (and not necessarily obvious, it seems) to many Brits, so I think there's a case to be made for also calling it idiomatic. Here's one comment on an idioms wiki - 'Of course the idioms wiki also has a flaw, namely the phrase "fit as a butcher's dog". I have no idea what it means. I might have to scour every idioms dictionary on the internet for it because it makes no sense to me at all'

And 'fit' in its sexual connotation, which is how I first heard this expression, is certainly idiomatic - In British English, 'She's well fit' has nothing to do with going to the gym.

At least I'm not alone in calling it an idiom; the excellent website for English learners, UsingEnglish.com, lists it as such.

Here are a few more as ... as ... constructions that seem to me to qualify as idioms:

as clean as a whistle
as cool as a cucumber
as easy as pie
as pleased as punch
as right as rain
as sick as a dog
as mad as a hatter

I have heard the phrase used, and used it myself many times, in both contexts, i.e. physically fit as well as attractive.
I found this thread, as I just used it with a work colleague who looked at me blankly so I googled it :)
I'm from Northern Ireland originally, but lived in London for just as long, and I'm now in NZ.