Distinctly possible!
> not referring to political parties and policy-making, but to the cultural zeitgeist ... the range of acceptable political thought and discourse
Hmm. I have never heard it used separate of policy. My understanding is it's the range of policies the public will accept and elect. If a proposed policy is outside the window then it'll be seen as extreme, or you'll not get re-elected. What usually follows are soundbites and helpful campaigns of politicians on message trying to shift public opinion first to render that policy acceptable. Sometimes this works (brexit, though I don't think they wanted to be quite that successful), often it doesn't. Put another way it's a way of gauging if "we'll get away with this".
So whilst said millennial progressives may be pulling back a bit from the accepted norm of neo-liberalism with free market as answer to everything, the individual being what matters (with as little government as possible), the pendulum after 40 years swinging right remains extremely far over on the right. If it's general political discussion outside of any reference to policy then the waters get far muddier with some aspects distinctly more left than others, though often not classic political areas of interest. It probably gets harder to judge the general view too.
Other than on the fringes there's little real discussion of adequate social housing, more public services, government providing safety net, society and community, failure of privatisations (except rail thanks to Corbyn) or even alternatives to austerity when after a decade it clearly isn't working. Mind that was likely a policy to hasten the reduction of government.
UK policy has always been rather reluctant compared to Europe on social matters preferring to stigmatise and pretend it isn't happening. For most social policies, cannabis included, the changes seem inevitable as the younger generation of both right and left are far more accepting. Even my generation seems often more relaxed than policy. I'm actually surprised it hasn't been legalised already - it's more common than tobacco now, and I've thought it inevitable for ages. My generation can't all have reached 40 and forgotten how they spent their youth!