They can only say 'Yes' or 'No'. And even if they say no, the fact that you took the time out to write to them before hand will show that you are not just keen, but proactive too.
I wrote to ULAS during my A Levels, before applying for a Bursary, asking if I could visit, speak to the studes, and *fingers crossed* scrounge a flight (in the then Mighty Dog, before they were used on the AEF flights too).
They invited me to Benson for a nose round, and had a great time talking to the Cadets, and even ate in the Officers Mess. Despite not getting a flight, it was well worth the effort.