I'm afraid to say that your chances of obtaining a job with as few as 300 hours total time will be very slim.
I know of at least three people out there with thousands of hours instructing time, all of them with type ratings of various descriptions (Turboprops, Jets) and still haven't obtained a job with an airline.
However you do have your age on your side.
At the end of the day it helps to know people within airlines who can put good words in for you etc...
You won't even be able to apply to air taxi firms as you won't have the minimum 700 hours required for single pilot IFR work. Instructing is probably your best bet, but add another 5-6 thousand pounds for the FI course.
Sorry to be negative... on the other hand you may be lucky and walk straight into a job... but the chances are very slim.
Best wishes,
Charlie Zulu.