HTC Hero :: PAYG Internet Access Without Time Limit

I'm a very light mobile phone user (100 minutes per month at most, often much less, with a sprinkling of texts) and I expect to need intensive mobile internet access only sporadically. Since much of this will be while travelling abroad I don't want to get locked into a large regular commitment to a UK carrier.

If all I wanted was phone access (calls and texts) the obvious answer would be a standard PAYG deal where the value of each top-up stays on the phone indefinitely until I use it. However, so far as I can see, this kind of deal doesn't seem to be available for internet data access (except at ridiculous prices like ?1 per MB).

At reasonable rates per GB all I can find are "rolling" deals where I pay monthly until I cancel or one-off "add-ons" (for example from Three where I can buy 500MB for use within 7 days or 2GB for use within a month). In either case, any allowance I don't use within the period goes to waste. What's more the one-off add-on carries the risk of forgetting to look out for the end of the 7-day or 30-day period and getting hit by penal charges.