I'm a bit concerned about sending them through the post since I live outside the UK.
So therefore it is the UK CAA.
It has to be said as stated before - there is a guidance sheet attached which answers your questions - so next time I suggest you read ALL the paperwork before asking questions...:
•Passport or Birth Certificate only. Photocopies are acceptable provided that the relevant information is clearly presented and certified by the Head of Training (or authorised signatory) of the approved training provider.
• Existing holders of UK Professional Flight Crew Licence or holders of another JAR-FCL Pilot’s Licence can ignore this requirement.
Logbooks:
Computerised logbooks are acceptable, provided that these are submitted in hard copy paper format, and contain the relevant
information (as specified in the Air Navigation Order Article 28) with each page certified as true and signed by the applicant.
Medical - Must be original
Valid UK JAR-FCL Class One Medical Certificate. Holders of a JAR-FCL Class One Medical Certificate issued by another JAA Member State should either contact the CAA Aeromedical Centre at Gatwick or refer to the CAA website
www.caa.co.uk for details on acceptability and mutual recognition.
• Validity must be sufficient to cover the anticipated licence
Send it recorded/registered where signatures are required. After all that's how passports are sent to you normally - so there shouldn't be a problem.