I had this with my Tier 2 sponsorship and it was a total pain in the butt. Like you, it wasn't a choice as I was still a US employee on the US payroll paid in US dollars. I was a "test case" to see if they could get my USD salary deposited into my UK bank account but my company had not purchased the package necessary with JP Morgan (our company's bank) to allow this. As there were barely any expat assignments, this was not worth upgrading.
At one point there were 2 other Americans in the same situation as me. We all had a different way of getting our money over.
One had an HSBC account in the US and had his salary deposited there, he was then able to move it from his HSBC US account into an HSBC UK account, I believe this cost him about £40 a month.
The other had a high dollar valued US account that was set up in GBP. I looked into this but the minimum standing balance in the account had to be $100,000 (the co-worker came from family money).
I did it a down and dirty way. I do think I could have found a better solution if I had found this board earlier. What I did was withdrawal my salary from an ATM every other week (when I was paid) in GBP and deposit it as cash into my UK account. Then my money was there for paying my rent / council tax/ other bills. My company reimbursed me for all my ATM expenses, etc.
A few things to consider:
1. See if you can get protection against exchange rates. The dollar is performing well against the pound right now but this can easily change and will affect your buying power.
2. Ensure you are "tax equalized" and that your employer pays either all of your UK tax and you are only liable for your US tax. This should be a hypothetical tax calculation that is withheld from your US salary.
3. Ensure you are provided with tax assistance on both the US and UK side.
4. Look into Paypal. I may have been able to save myself time and money by using this as I was only transfering over smallish amounts at a time.
5. Ensure your US bank will raise your ATM cash daily withdrawal limit dramatically.
6. Ensure you have someone in the US who has signature authority on your account in case you need a wire transfer.
7. Remember that since you will not have a direct deposit going into your UK account you will have an extremely difficult time building credit here (really just in the form of a credit card in GBP, you'll be able to build using utility bills, council tax, etc.). You will have a very hard time getting a credit card in GBP which means foreign exchange fees when you use your USD credit card (again, ensure you are getting reimbursed for these fees).
8. Get your employer to assist in securing a rental property. You will run into the circle of not being able to open a bank account without a lease and not being able to get a lease without a bank account. You'll need your company to front the deposits, etc., to secure the lease and sign their lives away to get the lease to open your bank account.
I know there is more... just can't recall at the moment. It was an absolute pain and I am now getting paid in GBP and life is so much easier. But it can be done. Chances are you will not "make money" on the move as life is generally more expensive here, but the experience can be something money can't buy.