[wolfbln]

I was able to top-up most UK providers with a German credit or debit card. I'm not sure whether it worked directly with Vodafone UK, but the PayPal bypass surely works.

EE works with out-of UK credit cards as well as giffgaff. O2 UK only worked with one of mine (N26) which is distributed (but not released) in the UK too. The tricky thing with Three is that they want to check addresses. Even if you use Kwikpay and PayPal set up to the same fake UK address (who cares there is no registration), it was declined.

Generally, the situation is going to improve within the EU. From the end of 2018 all merchants are not allowed to discriminate where the payment source is coming from. This means when they accept MasterCard or Visa, they need to accept all MasterCards or Visas at least issued in any EU country. You can see the first signs happening: on the TIM Italia top-up website there is a tiny boy to check, when you are now allowed to pay with a foreign credit card.

What the UK is concerned, we don't know if these regulations will take effect there as they are bound to leave the EU in 2019 one way or the other. And continental providers still sometimes need a reminder of the rules....To cut a long story short: There is hope, that this restrictive policy is coming to an end - but we'll have to see if this going to be really true for everybody.