BritRail Pass would be my recommendation, simply because of the flexibility it affords. No peak restrictions and valid on Heathrow, Gatwick and Stansted Expresses.
$179* Standard Class Adult for 3 days Consecutive Pass. Great Britain
$229 Standard Class Adult for any 3 days in 2 months Flexipass. Great Britain
Other passes for differing periods of use are available as well as differing prices for Children, Youths, Seniors and First Class. There's also cheaper England only and LondonPlus passes. The LondonPlus covers the former Network SouthEast area and the
GWML▸ to Bristol. (Not Westbury though!)
Another consideration is a Visitor
Oyster▸ Card (
PAYG▸ ) if any travelling around London is being done. Available from ^13 (^10 credit plus ^3 activation fee). Price in USD for that would be $20.67.
Sources and where purchases can be made (I recommend 2 weeks before travelling to allow for delivery):
http://www.britrail.com/passes (You'll need a non
UK▸ IP address to get full details and prices. Or use a VPN client like I did! Obviously they cannot be purchased by a UK citizen living in the UK.)
http://visitorshop.tfl.gov.uk/English.htm*Around ^110 based on today's exchange rate. Pretty good value for 3 days unlimited travel.
