The o2 Travel deal applies to Pay and Go customers and allows relatively cheap data roaming in Europe.
Correct me if this is wrong, but I believe the o2 Travel deal - started out at £1.99 for 25MB, went down to 15MB and now is offering 50MB data for roaming in any of a list of EU countries and some others at that rate of £1.99.
I used this deal in the past when it was 25MB is it is the simplest possible way to data roam in Europe.
Looking at the Ts&Cs today (deal link), it now looks not nearly so simple though.
I am sure there will be experts along in a moment to provide more detail, but to me it looks like they've recently tripled the data allowance you get for £1.99.
PS I did the HUKD search to check this deal but the search threw up nothing on keywords o2 and travel