The Lenovo ThinkPad L series would be more durable than the Asus. It is has gone thru MIL-SPEC / Military-Specification testing which generally means it is pretty rugged like if you were to drop it 6 feet onto a hard floor it will still function properly. This is one reason why it is likely to be more expensive than the Asus. Additionally, you get business level international tech support. If you were to travel with it to England and you are having problems with the laptop you can call tech support as long as it is still under warranty.
Because it is a rugged design the Lenovo should not have any flex issues. Additionally, ThinkPads are known for having very good keyboards, not quite as good as they used to be when they were still using the old style keyboards. So if accuracy "keyboard feel" is important, then the ThinkPad is the way to go.
Personally, I would get the T series, but you already stated that is too expensive...