I work for an insurance company and we can email all documents within the hour of setting a policy up. We dont use any embossed paper or signatures on our paperwork so as long as you print a good clear copy of the certificate (i.e using a laser jet printer) then your documents will be accepted by the post office.
As for the comment about Swiftcover being cheap, they are cheap because they are an online based company only. If you need to discuss ANYTHING about your policy at all the number they give you is a premium rate number which is where they make their money