In answer to your question, according to “Determining a planning application” by Ministry of Housing, Communities and Local Government, the local planning authority must formally notify the applicant of their decision using a written decision notice.
So I think you should give the planning officer who dealt with your application a call to clarify whether they might send the Decision notice letter to wrong address (email address or postal address) or whether they have sent the letter?
If you have instructed an agent to submit a planning application, the decision notice might be issued to your agent so you could also check with them.
But please note that if you feel the decision of your planning application is unfair, you are able to launch an appeal. You must lodge your appeal within three months (this deadline is for homeowners – developers with major projects have up to six months).
I hope this may help you.