few things:

1. you can also go from EWR from Philadelphia by starting your trip at Philadelphia 30th Street rail. Just type "ZFV" as your starting airport, and you will see the "ZFV-EWR" leg as a United flight operated by Amtrak. you receive miles for this, etc.

2. to use a GPU you need 2 things: to purchase a W fare or higher and to have R inventory available in the BusinessFirst Cabin. Currently there is plenty of R availability from EWR. as far as connections via IAH or IAD, on 8/14 there is availability on UA 112 FROM IAD, UA 918 FROM IAD. On the return on 8/19 there is plenty of R availability on flights to EWR; as far as connections there is availability on UA 959 to ORD, UA 984 to IAH, UA 920 to IAH, UA 949 to ORD, UA 925 to IAD, UA 5 to IAH.

3. If you have trouble doing this online, just call the premier line explaining that you would like to book a flight to london from Philadelphia using a GPU and they will help you do it properly.