Yes. When you first visit the 'UK Gender Pay Report' area, you'll be shown a list of all the reports you've generated. (Initially, this list will be empty, of course.)
You can create as many draft reports as you need: you may need to check your data; you may have several sub-organisations that need separate reports; you may want to check your current data against historical records. It's all fine, and you can have as many reports as you need.
However, note that because of the legal requirements of the report submission, the Umbrella wizard will always assume that the Snapshot Date (i.e. the date for which the report is actually being made) is the currently applicable date. So, if (for example) you're processing historical data, you'll need to make that clear in the label you put on the particular report, and ignore the Snapshot Date that the wizard puts in.