Tell them you have booked and paid for a trip to Outer Mongolia to count penguins - and do they care to pay for it if they insist on you joining in June?
A very long time ago I cleared all the hurdles to join the RN as a Pilot. In February I put down my preferred entry date as September, as I planned to meet up with a friend and work in the S of France for the summer, polishing my French etc.
A few weeks later I got a call telling me they couldn't guarantee me a place in September, but that I could go in April. Ooh says a spotty AA, that sounds bad - I'd better go in April, so I did.
Imagine my surprise when talking to the September entry they'd all had the same pitch, and told the chap to sling his hook, and got in regardless. The truth was that there were free slots they wanted filled, and they used any tricks to achieve their aims.
Despite the fact that you have a bursary, I'd doubt if it's lawful for them to order you onto a specific course, but check your facts and the papers you've signed before making an @rse of yourself.
Edited for typos