Have you checked all the beams to see if there is a Registered Number and Tonnage? Should be in the format Reg No XXXXXX (6 digits, probably starting with 1 or 3) followed by R.T. X XX/100. It may well be on the main beam just forward of the coachroof.
This could then be correlated with Lloyd's Register to find if her original info.
It looks like she has been Part I Registered at some time. Check with the MCA. As Registration only lasts for 5 years it is unlikely that it is still valid.
To renew an unexpired Part I costs £49.00 (I have just done it); if you want to resurrect an expired Registration it will cost you around £120.00.
If you go onto the MCA web page , and go onto the "part 1 register" link, and ask for a Transcript of Registry, they should be able to help. The details will include name, designer, builder, all dimensions, engine and rig, and original owner and port of registry. It'll cost £32, mind.
I have an old paper copy of the register (1964, so expect your boat to be there). It is an alphabetical list so, with only the number to go on, it could take a while.