unrealistic in most cases as it will slow your connection or system to a crawl if its doing on the fly encryption, decryption.
If you are being prosecuted you legally are required under UK law to provide the encryption keys.
This is without mentioning that unless you have a high level of encryption of your hard disk when your computer is seized they will know what you have anyway

Lets face it as well the vast majority of windows uses do not understand the os itself let alone something such as blowfish encryption