Selected Answer
The easiest way is to open Excel, and then import the CSV file into Excel.
(I'm assuming that you are currently just double-clicking the CSV file and it is auto-opening in Excel.)
When you go to import the data into Excel and you get to Step 3 of the Wizard, set the data type as Text and then click Finish.
Try that and let me know how it works; if it doesn't work, update your question with a sample CSV file.
Update
I just looked at your CSV file and the numbers have been changed to scientific format BEFORE they were exported to CSV.
Open the CSV file in Notepad and you can see this.
Here is a sample straight from the CSV file, before it was imported into Excel:
White,25_Plus,Passport,1.60E+206,12.5
Asian or Asian British,25_Plus,Other,16R101,24
Other ethnic group,25_Plus,Passport,1.60E+102,67.5
Other ethnic group,25_Plus,Passport,1.60E+202,7.5
I highlighted in bold the problem numbers.
This means that the issue comes from the program that you use to export the data to CSV.