GPS and GPRS are 2 different parts of your system. One has no bearing on the other. Please don't comfuse the 2.
See:
GPRS GPS
You could have a faulty GPS chipset. If this is a new phone, then you should be getting a replacement. Assuming you have phone insurance with O2 or it's within the first 30 days of purchase. Also, GPS will rarely work inside. Once TomTom is set correctly, then go outside and you should see the error NO GPS SIGNAL. If you see this then your GPS settings are correct, and you simply need to wait for a GPS lock.
You can always get settings sent from the O2 website for your Nav unrelated issue of GPRS -
http://www.o2.co.uk/services/ota4/input1
However, if you can use MMS and browse the net using O2/O2 Active, then your GPRS settings are working.