According to an article in the Times, Orange will soon introduce data bundles for its customers. We expect this to be formally announced in a couple of days' time, but the current info suggests that Orange's flat-rate bundles will be a tad more expensive than T-Mobile's... unless of course Orange bypasses the tiered system that T-Mobile uses (i.e. you pay more for IM and even more for VoIP).
We'll let you know the full details ASAP, but it does seem that the operators are slowly cottoning onto the fact that, if they want people to use the mobile internet, they'd better emulate broadband's flat rate norm. Now, how about data roaming...?