It will print a letter using my WP, Textmaker, with no problem, however when I try to scan a document to email it advises "Scan to email application not found."
That is unrelated to "default printer", which is a concept that is heavily deprecated on 64bit Windows (from XP 64bit onwards, nothing new to Win10). Default printers hark back to 16bit Windows, all (true) 32bit applications should be using application managed printers and 64 bit apps must use application managed printers. 32bit Windows retained the default printer concept to an extent, because they supported the 16bit applications through a thunking layer. 64bit Windows does not support the 16bit Windows API.
So, back to your issue, it sounds like the scanning software has not installed properly/completely, or the email application - thunderbird in your case - is not defined as the default email handler.
Does the scanner application work to scan a document to a file? If so, I'd suggest the latter. If not, I suggest the scanner application part of the printer software has not installed properly, or you are using the wrong application (not sure if that printer uses the inbuilt Windows scan app, or the HP specific HP Scan app, or something else completely.