OK before I went and tested on O2 I had to reset my very old DG834G Version 1 router !
There is a tiny hole at the back of the router next to the first Ethernet port on the rear panel, you need to use a uncurled paper clip and press and hold for about 10 seconds and release to reset to factory settings. Watch the Test LED light on the front panel, while you are pressing the reset button for 10 seconds this Test LED will come on and then it will switch off. You do not need to reboot the router while resetting it !.

This is the advanced precision tool I used to reach the RESET button

Now that the router was reset,
I used the default login details “admin” for username and “password” for the password [obviously without the "" marks]

Now I was back in the router admin panel and configured the following to access O2 ADSL line

ADSL Settings
Multiplexing Method : LLC Based
VPI : 0
VCI : 101

Basic Settings
No user or password required
Does Your Internet Connection Require A Login : No
IP Address : Get Dynamically
DNS Address : Get Automatically
NAT (Network Address Translation) : Enable
Router MAC Address : Use Default Address

Now it was Testing time !
I tested on the Ethernet LAN using a cable and got a maximum of 2888Kbps as you can see below

Then I tested using the Wireless LAN and got a maximum of 2888 Kbps as below.

Was I happy with these speeds ? NO !, the Thompson router (given by O2) gives about 4 times more bandwidth !
Anyway it was an interesting experiment and I have another ADSL router in case if I need backup.

If anyone needs help with setting up a router with O2 or any other provider leave a note below and I will try my best to help.