Caity is spot on.
I've just returned from a European trip and found the wifi in most hotel to be great - fast and unlimited!
There is also wifi on most of the TopDeck coaches these days, but be warned the allocation is quite small and many people run out very quickly in 3-4 days!
You can get a good deal on a data SIM even before you leave home - I got one for about $50 AUD that gave me 12 gb to use over a month and worked in every euro country I visited as well as the UK. You can get them posted here in Aus for about the same price as you pay after you land in the UK/Europe.