I have BTanytime and you can use it from any phone in the UK, providing that your home machine is not also logged on. If it is you get a snotogram from BT about multiple users and a threat to cancel your account if you persist.
Regarding GPTN and freephone access:
1. In the UK, you can get to any freephone number by adding another zero before the 0800, 0808, 0500 etc numbers and that will connect you. (However, you have to add another dial up connection to reflect this new number using your original user name and password ).
2. From overseas locations I'm not sure if it connects through GPTN.
3. Connection speeds are usually slow due to the nature of so many shared lines and distance from the main exchange.
Hope this helps!