Its my 18th birthday in September, and my girlfriend has said she wants to get me a guitar, but knows very little about guitars, my girlfriend doesnt earn that much so ~£300 is the limit atm

I must decide from the following:

a Dean Cadillac
a 12 String guitar
an Ovation guitar (an ovation 12 string guitar?)

Any recommendations?

A 12 string guitar sounds cool ! Its very fun to have

Ive played a few, they sound soo sweet compared to normal 6 strings

And ive heard Ovation is the best you can get, so i wonder how an Ovation 12 String sounds

12 strings are great until u have to restring them haha

Well, see if you can even find Ovation 12string models under 300pounds. THey are expensive. In the end try them all and see what type of sound fits you the best.
The 12 string guitar is good choise, you can put only 6 strings when you don't need 12, but this requiers adjusting the truss rod in the neck. Also be prepared for double the time spent when restringing it. Make sure you get a quality one, cause they get out of tune more easily, and they are pain to get in tune without a tuner.

ovation nice guitar

Hahaha can't agree more there

But 12-strings DO sound really sweet

+1 I would go for the 12-string in this case