You may want to try topping up using a electronic top-up purchased from Kwikpay (kwikpay.co.uk) as they will accept PayPal and USA credit cards.
I prefer using PayPal as my credit card company always seems to make me call and verify the charges whenever I'm billed by Kwikpay, which is a hassle. (I suspect Visa is very vigilant when it comes to small charges billed from overseas when I'm at home and have not notified them of overseas travel).
The only slight complication is that you MAY need to install the Kwikpay App on your smartphone or tablet. It is supposed to work through their web site directly but logging in there often doesn't work for me. Their iPad/iPhone apps has been trouble-free.
P.S. Just processed a 5-pound top-up for my Tmobile-UK SIM card. It worked correctly, using the Kwikpay app.
Last edited by BrentHutto; Feb 5, 2016 at 12:52 pm