I can get a browser to the internet
To be clear ....... does that mean that you can access the internet without hindrance using a browser?
You can access all sites you have tried?
What happens if you copy the URL from the entry in Synaptic for your chosen mirror?
Can you access the mirror through your browser?
If you can't access the mirror with the browser, then please try the URL from a different mirror and see if your browser will access that one.
Let us know what happens in each of the above cases.