//I didn't see any evidence of it amongst the Civil Service.//
There's been stacks of reports explaining how the Civil Service - from the top to the bottom - was, shall we be kind and say "less than enthusiastic" at the idea of Brexit. Here's just one example:
Unfortunately it's behind the Torygraph's "paywall", but you can get a flavour from the bit you can see:
https://www.telegraph.co.uk/politics/2019/03/18/believe-civil-service-trying-sink-brexit-have-seen-inside/
"As a civil servant I can tell you large parts of the Whitehall machine are systematically working against leaving the EU. I have met thousands of civil servants in the past few years: I can only recall five who voted for Brexit. At first, I thought they were perhaps just staying quiet given the political climate, but my worst fear was confirmed during the high-profile remainer Gina Miller’s successful court case to make sure Parliament has a say on the Brexit outcome.
When it was announced she had won her case, I witnessed large teams within the Foreign Office break out into cheers and applause."
Of course their motives for not wanting to see the UK leave the EU may be many and varied. It could simply be that they were not too enamoured at the thought of having to undertake the work that their colleagues in Brussels and Strasbourg have been doing on their behalf for the past forty years. Though I suspect their motives are more likely to be a purely ideological.
There are plenty more reports around in a similar vein. There is no doubt in my mind that the CS was staunchly anti-Brexit and that the Mandarins would do all hey could to either thwart it completely or at the very least make the version finally agreed upon to be scarcely any better than remaining. They were in a unique position to do just that as their advice to Ministers was clearly based on a "worst-case" scenario with little or no balancing information surrounding the benefits of Brexit.