Really ? Do you have evidence to support this or is this just a gut-feel ?
If politicians in the UK were so afraid of a lack of tax revenue then they wouldn't plough so much money and effort into anti-smoking initiatives and wouldn't be celebrating the (relative) success of those schemes. If politicians really were concerned about the tobacco tax spigot running dry then they wouldn't have stopped advertising, stopped displays at point of sale in large shops and wouldn't be in the process of introducing legislation to standardise cigarette pack design along Australian lines.
UK tobacco revenue appears to be steady or even rising even though consumption is down. link
Perhaps the politicians are managing to have their cake and eat it?