If you are low usage, I'd go with a Virgin Mobile Pay as You Go SIM activated on direct debit. The rates are reasonable and if you activate direct debit, you get international roaming turned on and have most of the benefits of a contract phone with no commitments. If you go with a contract plan in a couple of months, shelve this phone and don't port the number. You can keep the Virgin SIM available for as a fallback with very minimal usage.

What plan you want to go with really depends on your calling habits and you need a little bit of time to truly sort that out.

The roaming SIM described by someone else is a nice addition to your tool kit, but I probably wouldn't use it in the UK unless you are really low usage. The phone uses a callback process for making calls. You dial the number and it automatically sends an sms to triggering an incoming call to you. Your phone number is not a UK number but an IOM number. On many tariffs, this will be treated as a UK number, but not all. I have a SIM from globalsimcard.co.uk and like it.

For the US, you can get a prepaid SIM for T-Mobile relatively cheaply off e-bay. You can forward the travel SIMs to that number without a charge or just give that number out.

I just left the UK last week for Dubai. (I live in Dubai but frequent the UK). I left a message on my UK mobile simply saying, "Hi, I'm no longer in the UK. Call me at my Dubai mobile 00-971-50-XXX-XXXX and that seems to work fine).

If you are packing a notebook in the US, don't forget about something like Skype. You can call the UK really cheap off it from any room equipped with Broadband. Also for US$14 you can get unlimited calls to the US off Skype.