TTX just doesn't accept foreign currency payments, or CC's without a valid U.S. address/phone tied to them, due to the high amount of fraud experienced in the past. Now the below information is based on my experience when I lived in England about 30 years ago. But it's all I have.
The US Embassy in London, and most of the consulate offices around the country have US banking facilities in them. Locations of consulate offices in the UK can be found at https://uk.usembassy.gov/embassy-consulates/
If possible, contact the one closest to you to confirm they have a US banking facility in them. Then you can purchase a pre-paid VISA in US dollars for whatever amount you need. (They'll even mail it to you via Royal Post once your over-the-phone UK Credit Card payment has processed.) After you purchase it, you'll need to go online and register it with a valid US address and valid US phone number. It doesn't necessarily have to be your US address and phone. It just has to be "valid". It can take anywhere from a few minutes, to a few hours for the internet registration to propagate before the card will actually work online.
For a valid address, 1600 Pennsylvania Ave, Washington D.C., 20500 works just fine. Or at least, it did for me back in the early 90's. 🙂 I was living in Biggleswade about 40 minutes north of London by train, and it was no problem for me to get a prepaid VISA back then. Even paid for it with a check in pounds, on my UK banking account.
