Hi There
The UKBA website states "You can apply for your visa within three months of your intended date of travel. We are only able to post-date visas by a maximum of three months from the date of the decision on your visa application." Which I think is crazy given that in some places it can take longer than 3 months to process. It is also recommended to travel within 3 months of the visa date. I think this is due to the length of time you must been in the country before applying for further settlement visas, although I am not 100% sure about this.
We applied in October and put a date for end of December. When they stamped it they put the date of when they approved it, mid November, disregarding the date we had given, which I thought was strange.
Our application was also in the Caribbean and only took a few weeks!! I hope you have just as much luck