We went through this with the titan on sprint.
Titan - no.
I brought my AT&T Tilt over, and it worked great. It did have a "native" O2 sim card in it at the time.
Also used an off-the-shelf HP/ipaq navigator (WM5!) which took forever to lock, but also worked.
So aGPS, not so much w/o signal, even in standalone mode
And yes, you'll want to buy those UK maps.
It is VERY worth it to have SOME kind of GPS there. Street signs are a joke, and you really want to be focusing on "STAY ON THE LEFT!!!!" and not a map. A GPS that says "second exit" when you go into the rotary is worth its weight in gold.
Oh, wait. if you are going to LONDON only, forget the GPS. Get a Tube map. Forget the Car.
Tube + walk. It's I think 18 pounds for a 3 day pass. Well worth it.
Enjoy the trip
