I have been looking for what SAGE documents I can find. This the latest I have found so far (16th March)

This recommendation caught my eye (emphasis is mine)


4. It was agreed that a policy of alternating between periods of more and less strict social distancing measures could plausibly be effective at keeping the number of critical care cases within capacity. These would need to be in place for at least most of a year. Under such as policy, at least half of the year would be spent under the stricter social distancing measures.


I would say SAGE did not seem to think much of reopening quickly.
I accept there may be later reports that change this view.