I thought that my experience might be of interest in this discussion.
I'm with TescoMobile, which uses the O2 network.
When we're at home, we use very little non-WiFi internet, but we go away for 6-8 weeks at a time to Europe in our 'van, a couple of times a year, and then our usage increases significantly.
When my last contract expired, I waited to see what the different networks did with regard to roaming charges. I saw that O2 and Tesco were not charging for roaming, so I took out a two year contract, hoping that if things changed I would at least get free roaming until the end of the contract.
As I had occasionally had to buy extra data while travelling (and 'extra' data is always expensive), I went high: 30 GB for me, and 12 GB for my wife, on a 'family discount'. Also, when I had taken out a previous contract, I was given a free 6 month trial for Tesco Plus - this gives 10% off two shops per month plus a few other discounts, and *double data* on one phone account.
The Tesco Plus now costs £7.99 per month, but it is not difficult to get two shops to total more than £80 in a month to recoup this, so it is effectively free (unless I'm out of the country for the whole month, which happens occasionally!).
So, for £20.50 per month we get 12GB plus 60GB, with free roaming in the 'Home from Home' countries. I think this is pretty good value, especially if you use Tesco anyway.
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