Originally Posted by
WorldLux
In theory, yes. In practice, some providers have restrictions in place (e.g. basic contracts that can only be used domestically). Even if the contract allows roaming, providers are still able to restrict the free roaming via fair usage policies. Reading the fine print will provide more details re: usage within the EU.
I've been disappointed by my last mobile phone providers (Three UK being one of them). All three significantly restricted 4G usage abroad, offered only low speeds and/or not connecting to networks despite good network coverage.
As WorldLux says, you need to be careful of the fair use policies. But there is a difference in what is in the T&Cs and how it is enforced!
If you use the SIM card first in the UK for a few days and then go to France then it should work no problem. But if you try to first activate it in France then this probably would not work.
Also, different operators have different ways and methods for monitoring and enforcing policies. So some might recognise that you did not spend much time in the UK and cut you off, while other might only check every few weeks. Although newer SIMs are closer monitored in the beginning. They might judge this based on your usage pattern or by the number of days domestic vs abroad.
If you are in the UK, then it might be an idea for you to already buy the SIM cards a couple of months in advance. Activate them, make a simple call every month, and then give them to your friends when they arrive. That way, you have built a "history" of usage in the UK so it might look better.