Suggest that you buy a copy of LASORS from the CAA at the cost of £5 - this should answer most of your queries.
You can only add a Multi Engine Piston Class Rating or a Type Rating to your 'UK' PPL by completing JAR-FCL requirements of training and testing. There is no such thing as a 'CAA Multi' - presumably you mean the old Group B rating?
If you're not going to use a multi engine rating in the UK, then why bother? It is very expensive to maintain proficiency and to revalidate your rating as the rules have changed substantially under JAR-FCL........