Their terms seem better than many others.Further to my earlier post ASDA has now shifted to the Vodaphone network.
PAYG bundles can be used in most of Europe as at home, according to this:
"Roaming with Asda mobile
With Asda mobile, you can use your selected bundle in 36 European destinations the same as you would at home in the UK!...................... "
Of course this may not suit heavy users but that is a different matter.
Looks as if you'll get away with a 4 month period at most. Of course that might be 2 months in UK + 2 months in EU to trigger that, which is a more normal limit. However, they then talk about proof of normal residence (e.g Utiliity bill). That sounds better than most.
When roaming in our European region you will be able to use your bundle allowances at no extra cost. This "roam like at home" facility is available to anybody who is normally resident in the UK or who has stable links entailing a frequent and substantial presence in the UK. If, over a 4 month period, your use of the service suggests to us that you may not meet this requirement we may ask you to provide reasonable evidence of your normal place of residence and of your frequent and substantial presence in the UK. Examples of such evidence might include an address and utility bill, evidence of UK employment or proof of registration with a UK public authority. If you do not provide such evidence within a two week period after our request then we may, without further notice, start to apply additional roaming charges (specified by the European Commission) for your Services (see your Price Plan for details of these additional charges).