Hello and welcome to the Community, Muz981
We'll be happy to share some guidance on the features available in QuickBooks Self-Employed UK.
Currently, there is no option to set a customised email template, and the default would need to be edited each time.
The option to customise the default is already available in QuickBooks Online, but we recognise your current plan may fit your other business needs better.
If you would like to leave feedback so that this can be considered by our Product team, please consider sharing your idea over on QuickBooks Idea Exchange. We'll be around if you have any further questions!