Find Your Cruise
It really depends on what you want from the cruise. Whether you want a port intensive one, in which a fly/cruise to the med or caribbean is probably best, or if you want a more relaxing one with more sea days, then sailing from GB is best.
The length of time you can be away will always have a bearing on the cruise you go for.
Honestly unless you really have a more specific idea of what you are aiming for I don't think you could do worse than contact a cruise agent, try the freephone number at the top of the page, they will be better placed to help.