I have successfully applied and made payment for UK visa around 26th February. Intended date of travel I wrote on the form was 4th April which I wanted to change to mid-May but just could not because once the form is committed one can't change it. To no avail I also tried contacted the visa office and the British High Commission. Now my appointment date is 8th April, can I make a requisition for changing the date as I will not be able to travel before mid may and I would not want to risk if they issue me a visa for three months from my intended date of travel then I may not be able to make proper use of my visa and travel at all..
For the sake of an answer, to repeat a Comment from @Gayot Fow:
Very normal. After you print out the form you cross out the date with a pen and put a new one above it.