I have sold and putrchased guns over the last few years and just emailed the appropriate information to my local force ( Essex) . I always ask for confirmation which when received can be printed off and produced if necessary..Some years ago I sold a gun and sent the details by recorded post .I only found out this letter had not been received when the firearms officer came round to inspect my guns (,when my certificate was up for renewal).I had the certificate of posting and the details of the guy who bought the gun so there was no real problem.Just supply the same information as you would in a letter.