According to the graph on page 23 of the document, the Delta variant only started becoming dominant as from the middle of May. During the early days that you are speaking about, there was only the Alpha variant, and a small bit of the Beta variant during April. These variants have no relevance...
Here is another way of looking at the same UK data. With reference to the document: "SARS-CoV-2 variants of concern and variants under investigation in England: technical briefing 18" that can be found at the following link...