Unforseen consequences
Suspect (as some articles on new roaming rates here already suggest) that the result will be that mobile networks will implement this by setting the cut-off at £0 and offer to sell you access at at a "per day/week" rate with a bundled data allowance. So, if you away for a week and wanted to be able to occasionally lookup something on a website then you may end up having to spend £30 on a 7-day 30MB bundle when in reality you only wanted to be able to download a few 100k on 2 or 3 days during the week.