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Cheapest USD to GBP transfer 
Use a remitting service such as Ria Money Transfer. Almost all these services allow you to transfer upto $2999 at a time. So, you would be able to transfer the entire amount of $4500 within 2 business days(There is a monthly limit too, but it will definitely be more than $4500). There are no fees to use these services, but they do scrape off a bit on the currency rate. As of today you are getting 624 GBP for $1000 whereas the market rate is $641.95. You still save roughly $17 and 4 transactions, which adds up to more than $100. Here is a link to Ria's website. Other services, include Xoom, Western Union, Money Dart and Money Gram.