Thanks for the view both, definitely change the plan to the Washington Flyer then (I'd misread the 5A as being the better option for some reason).

Uber would appear to be a bit different in the UK from the US (haven't tried it "over the pond" yet). From Heathrow its become my preferred option for being approx 30% of the taxi price, sometimes you get a nicer luxury car and I can just pay directly with a credit card rather than cash. However that is a bit of a backup option.

Pentagon was just to see the memorial actually (last time I was there was probably 2006, though I've been to Washington last year), will see how time is going as the national cemetery is something I'd forgotten about and if its a nice day is appropriate too.

Thanks for the tips EricTheNerd, I'll save down to use if I get the chance. If there are any other tips on food I'm easy - something "American" perhaps for a reasonably pleasant lunch, perhaps a nice chunky sandwich and chowder for example. (Was in Mexico over Christmas tasting "real" tacos)