I was looking into this this morning. I'm looking at migrating next week and asked the question on keeping my email address. If migrating away you'll need to request that your account be downgraded to a 'dial up PAYG'.
Looking at the features of the dial up PAYG product it comes with 25MB of webspace as opposed to the 250MB you get with the paid products. I'm assuming that if you downgrade (allowing you to keep your account open) you also get to keep the webspace but limited to 25MB.
Best thing to do though is to ask the question in the ticket to CS.
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Tulse Hill Exchange LSTUL