UMTS speed is 384Kb/s at best. The distance you are away from the transmitter affects the speed. Realistically you probably can hope for around 300Kb/s. HSDPA will increase this max speed to 3.6Mb/s (Downloads) but this has yet to be installed thoughout the 3UK network and is also handset dependent.
The speed is not too important for handset based applications. Video streams for small handset displays can be very watchable at less than 100Kb/s.
Should be perfect for viewing YouTube etc on the phone. 3 don't allow modem use with any of their add-on's including x-series and that is when the speed really counts!