The WiFi on the A330s is pretty decent, certainly adequate for browsing, sending iMessages, etc. WiFi is complementary for Business Class passengers, and chargeable in Economy (€8 for an hour, €14 for the flight - I think its actually 24 hours pass). As it is a satellite system it works reliably over the atlantic, I didn't suffer any noticeable problems. Im not sure if Spotify or similar would work, I think youTube is blocked.
Considering UK Train operators charge £4 an hour to use 3G WiFi (the system is provided by EE's 3G network - i.e. the network your phone already uses), I don't think EI prices are unreasonable, given that you are hurtling through the sky at 500mph and don't have a lot of other options.