牛肉を1ポンド分、下さいって英語でなんて言うの？

In order to be specific about the number of pounds you require you can say:
Can I have (...) pound/s of beef please?
Can I have 2 pounds of beef please?

If it is only one, you can say:
Can I have a pound of beef please?
Can I have one pound of beef please?

I hope that helps!

You can ask :

May I have one pound of beef please?
Can I have one pound of beef please?
Please give me one pound of beef.
I would like one pound of beef please.

>1 pound of beef please.

*This sentence is the best and clearly indicate how much you want and what you want.

The weighing of items in 'pounds' in the UK has been phasing out for many years and now is becoming quite a rare occurrence.
A kilo is equivalent to 2.2 pounds (lbs) and so you may well ask for 'half a kilo' of an item which is an approximation of one pound in weight.
"Pound of beef please."

If you mean a 'pound' in money terms, then you say:
"Could I have a pound's worth of beef please?" so that you only spend 1 GBP.

1.Can I have one pound of beef,please.
- You can structure your sentence like this. However you can also specify how many pounds you want.

For example: one pound,two pounds or 3 pounds.

2.Can I have one kilogram of beef,please.
- In my country we use kilograms instead of pounds.

When talking about measurements then a kilo is just over 2 pounds in weightso as well as asking for a pound of beef you could also ask for 'half a kilo of beef'
to be polite always add please at the end

To ask for 1 pound of beef, you can say:
"Can I have one pound of beef please?"
"May I have a pound of beef please?"