However, if I was going to cheat I'd probably go for a tub of Bisto White Sauce Granules, largely because (unlike the packets) they don't seek to tie you down to making a specific amount. £1.85 in both Tesco & Asda.
PS: You can make up the sauce fromthose granules using just water but it's far better made with milk.
White sauce is dead easy in the microwave - just put all ingredients into bowl and whisk like mad at 40 second intervals until it thickens. Then microwave gently a bit longer. Job done.
If your making a pie I'd just add the meat to the pan after sauteing (sp) some onions and veg etc, add a couple tablespoons of flour, stir, then add some stock. Mix well until thickened.
jenny ignore posts that say 'its easy' -things are only easy if you know how to do them and have the skill! Any packet sauce can be 'souped up' by using half milk half cream and if it says 250 ml use only 200 it makes it much thicker which you will need for a pie. For a chicken and bacon pie I would add a teaspoon of mustard and a little dried or fresh sage.