You could make one using Microsoft Word but you would also need a printer that can handle envelopes. In fact you could use any graphics package to create your design. The majority of FDCs are based on the DL size envelope and Royal Mail will tell you how much white space they need to print the 4-state barcode and where on the envelope it will be printed.
Do you know that we're all in line for succession to the throne? Really? Well, if forty-eight million, two hundred thousand, seven hundred and one people died I'd be Queen.