We have no idea how many caught it in March/April though, we only know how many tested positive, from relatively low test numbers. Now somewhere around 1000 per day are testing positive from generally >100,000 tests per day (so around 99,000 are testing negative, many of who must of some symptoms).
The virus is just as infectious (see the sandwich company in Northampton), but less are being infected because socially we are all behaving differently.
Also we now have a better idea how to treat those that get the more serious infections, and they are spending less time in hospital, and less are getting the most serious illness. None of this makes the virus weaker, but it all adds up to make it look weaker.