SS deposits directly into a UK bank account are at the best possible Fx rate on the day, with no spread, and no fees. By contrast, every time you use almost ANY US credit card (or debit card), you attract a fee - typically 3% - ad the Fx rate is not that great. Cash withdrawals are even more costly; I wouldn't even do that in the US with a credit card.
personally, I would never use a US card for a purchase here in the UK. Also, remember that your US SS is taxable in the UK (90% of it, not the full amount), not in the US under the tax treaty. Having it deposited in a UK account means you know exactly what to report on your UK self assessment tax return.