Is UK Visit Visa Given Only Until I-20, F1 US Visa Ends?

[ESCO]

I am an international student in the US with a valid I-20 and F1 visa.

I have been studying in Ireland before I moved to the US to continue my studies. In my period in Ireland, I had 2 year multiple UK visiting visa (which has now expired) which I used to go and visit my sister (this was however obtained from Ireland).

As UK 2yr visa has now expired, I now want to apply for yet another 2 yr Multiple Visit Visa to the UK to visit my sister during holidays.

My F1 visa however expires June 09 and we are currently in Aug 08.
Will the UK embassy officials give me a visa only up until the time my US visa is valid, OR Will they give me the 2 yr visa since it states on my I-20 that my study in the US ends in 2011?

Has anyone been in this same situation, what did u do?