I have found a rate calculator on usps.com. Here is what it will cost sending envelopes to:
Great Britain and Northern Ireland
(Includes England, Scotland, Wales,
Northern Ireland, Guernsey, Jersey,
Alderney, Sark, and the Isle of Man)

Airmail
Letter-post Rates

1oz. $0.80 12oz. $7.55 44oz. $16.90
2oz. $1.60 16oz. $8.70 48oz. $18.10
3oz. $2.40 20oz. $9.85 52oz. $19.35
4oz. $3.20 24oz. $11.00 56oz. $20.60
5oz. $4.00 28oz. $12.15 60oz. $21.85
6oz. $4.80 32oz. $13.30 64oz. $23.10
7oz. $5.60 36oz. $14.50
8oz. $6.40 40oz. $15.70


Weight Limit: 64 ounces (4 lbs.)
Just to let you know say you have a package that is 1.1 oz. then you will be paying the 2 oz. price and so on from what the website says.