Many pubs (particularly the big chains) have taken the simple option of banning them.
e.g. from the J D Wetherspoon website:
"The issue of whether or not electronic cigarettes can be used in our pubs has been considered at the highest levels within the company.
We have also, on occasions, experienced customers attempting to pass real cigarettes off as electronic cigarettes, resulting in customers actually trying to smoke in the pub. Although these incidents are rare, they can result in the pub being heavily fined.
Therefore, due to the problems experienced with the monitoring of these cigarettes, we can confirm that we do not permit the use of electronic cigarettes in our pubs "