Once your wife is in the UK, having her SS deposited to a UK bank is easy. Under the UK-US tax treaty, her SS income is only taxable in the UK, and its 90% of the total SS amount, not 100%. Having it deposited to a UK account eliminates questions and conversion calculations, as it is deposited in GB£. The Fx rate is the best you will get on the day of deposit (not a 'tourist rate'), and there are no bank fees involved on either side.
If she has no other income, her UK self-assessment tax return is straightforward - very easy to do. She should continue to file a US tax return, but the taxable amount of SS will be 0.