There are commercial scanning services which will scan A3 documents for you but these are often very expensive.
The simplest way to do what you require is to head for your local library where they'll almost certainly have a photocopier that can reduce A3 documents to A4. Then you can simply scan them ourself.
Assuming that your property is registered at the Land Registry (most, but not all, properties are), your paper deeds are largely redundant anyway. You (or anyone else) can download the key documents, for a few pounds, from here:
http://www.landregisteronline.gov.uk/
Chris