wire is your only option of those three:

1. for USA->UK wire transfer costs $20-$50 and takes a few days. it's a huge PITA getting all the data correct for a wire transfer, take extreme care.^†

2. ACH is ONLY for within the USA so irrelevant to you. (it is free and takes a day or so, but has no connection to USA->UK)

3. "IAT" or "Global ACH" is not available these days so forget it. it's bizarre they mention it

Of the three mentioned wire transfer is your only option

Here's a perfect article explaining:

https://transferwise.com/us/blog/international-ach-transfer

nobody uses wire transfers these days, just use a transfer service:

Other options:

1. if it's a small amount of money (under $500 say) inevitably it's easier to just use PayPal or similar in some way (fees will be very high)

2. What everyone does these days for international payments: just use one of the two major international payment companies, OFX or Transferwise.

• you do have to set up an account which is a bit annoying (however, setting up an account is much easier than making just one old-style wire transfer). once you set up an account you're set for life

• once the account is set up, it's extremely easy to send money USA->UK. it takes seconds

• it is far faster and cheaper than wire transfer.

• more importantly, it's the best possible exchange rate (dollars to pounds) you can get

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^† this site is flooded with questions "my wire transfer went missing, what to do..."