What's the equivalent bet to laying at a bookie?

Yes, double chance - team x + draw, though I assume laying team Y on an exchange is generally more efficient.

Asian Handicaps or Straight Handicapp Betting

Great, thanks

So if, for example, I want to lay Man U in Man U v Arsenal, would I bet Arsenal +0.5, giving me a win if either Arsenal win or it's a draw? What's Arsenal +0.25/0.75 etc?

Yes you are correct on the 1/2 goal market but Arsenal were 6/4 plus the half goal so why bet with the high street gang when you just lay Man utd at 1/2 on here means your are backing the Away team at 2/1 instead of backing a 6/4 chance

Just lay on here.  Backing +0.5 or doing the double is far worse odds than laying on here would be in most cases.

To answer the question about Asian Handicaps, Arsenal +.25 means half is settled at +0, so draw no bet and half at +.5, so you win if Arsenal does not lose. And +.75 is half at +.5, sp no draw no bet, and half at +1, so no bet if Arsenal lose by exactly one goal.
Its quite hard to understand at first.