Go for the training now.
19 is not too young, in fact there are more 18 / 19 year olds now on training courses than ever before.
Airlines cannot be ageist any more its the law (EU). Some still try though.
And wasting 3 years on a degree course when you could be in the Right seat of a modern jet after 20 months is the right way to go, especially after completing an integrated course at Oxford which now includes a degree.
Go for assessment and get ready for a great career.
I know of a 19 year old in Titan Airways and an 18 Year old in BA.