specify variable context path in spring security (3.0.x)

We are developing "box" web-product, so I cannot foreseen the environment where application is deployed. This application uses spring-security. But it is appeared that if Tomcat placed application not at the root then following configuration fails:

<sec:http auto-config="true" >
    ....
    <sec:form-login login-page="/ui/Login" />

Following recommendation: https://stackoverflow.com/a/6934712/149818 to use

#{servletContext.contextPath} doesn't work. User instead of http://foo/myApp/ui/Login is redirected to http://foo/ui/Login

So my question is

either how to get #{servletContext.contextPath} alive
or how to force spring-security recognize actual contextPath

Solution

Great thanks to @Luke Taylor and @axtavt - my error is missing root '/' in the

<sec:http auto-config="true" use-expressions="true">
     <sec:intercept-url pattern="/ui/Login" 
                        access="IS_AUTHENTICATED_ANONYMOUSLY"/>

When '/' appears - #{servletContext.contextPath} starts work correctly.