Given the devastation caused by a relative handful of U Boats had 300 been available which triples or Quadruples the number of boats at sea at any given time over OTL - and this will suddenly massively up the number of ships being sunk beyond a sustainable number month on month which means that the British have to react and react fast.
The quickest thing the Brits can do is retask Bomber command to coastal command (which is what they should have done OTL) given the impact a relative handful of aircraft had on the BOA suddenly having scores if not hundreds operating across the western approaches and as far as possible covering convoys is goign to cause the Uboat fleet many problems.
If the Germans can keep 4x as many Uboats at sea over OTL those voices will no longer be loud enough and the resources of BC will be transferred to Coastal Command
Then of course curtail any large warship construction and concentrate on building as many escorts as possible and training crews accordingly.
OK but there's a potential problem. If tonnage losses far exceed historical levels early in the war, the British will have fewer resources with which to implement alternative strategies.
Also I do not believe that the Germans building 300 U Boats by 1940 is achievable - recall that they had only started making them again in 1936
They had made relatively few down to '39 yet within three years, by the end of '42, had about 200 operational--despite the loss of 86 in '42 alone.