Three has confirmed it'll be trialling ad blocking for one day in the U.K. during June. This will be network blocking, and not user blocking. A selection of Three's users will be given the opportunity to try the ad blocking in the one-day test.
Three said it was exploring ad-blocking on its network because:
advertisements count towards customers' mobile data charges, rather than advertisers shouldering the cost
some advertisers "extract and exploit" customer information
customers do not always receive relevant adverts and have their browsing experience "degraded by excessive, intrusive, unwanted or irrelevant adverts"
U.K. Mobile Operator, Three, Confirms Ad Blocking Trial [bbc.co.uk]
Mobile giant Three to block online advertising [webmasterworld.com]