Another plagued soul...
For what it's worth, I experience the same problem under the same circumstances - Razor plus fully patched 3D client means no dice, Razor continues to respond normally while the client itself ceases to respond at the "Entering Britannia" screen.
And to confirm MrFixit's statement, connecting to the server with Razor's interface, cutting UOGateway out of the loop, results in no more success.
For what it's worth, there is another abnormality that accompanies the client freeze, I don't know if others experience this too: when the client freezes at the "Entering Britannia" screen, it is accompanied by a (subtle) graphics glitch - a discolored rectangle within the loading screen that's not normally there. I don't know if Razor makes any modification to the display of information within the main client window itself, but to someone more familiar with the Razor code, maybe this points to something in particular as the culprit behind the incompatibility.