The one supposed benefit of a rapidly devaluing pound was that it was supposed to make Britain's exports cheaper. Yes, let's forget for a moment that all imports get more expensive, including those raw material imports used to create manufactured exports, and let's forget that a Pound worth zippo does not mean superb export growth, but actually means terrible poverty. Let's also ignore the record of all the highly successful export countries over the last few decades (e.g. Germany), which have generally had stronger currencies than elsewhere.
Let's go ahead then and swallow the ridiculous line that we should encourage and be happy about a devalued pound because it's the only way for us to get richer. (To say it is to expose its stupidity.)
Having swallowed that bilge, where do we go next when it turns out that a devalued pound has not grown exports, at all? But that exports are stagnating, and that imports are still rising?
Keynesians can answer on a postcard.