I've just checked for rates when going to Spain later this month and found out, that Toggle has launched (finally) roaming packages for their Toggle countries!
http://www.togglemobile.co.uk/roaming-data/en
1GB for GBP 20
2GB for GBP 30
3GB for GBP 40
This (if it works) is a very good reason to stay with Toggle!
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Mobiles: iPhone 7+, iPhone 7, iPhone SE, Blackberry Priv, Honor 8, Lumia 950
Prepaid-SIM: 3 UK, Save Money Sim, Toggle Mobile UK, XX-Sim, Koko Mobile