We are not due to have elections for nearly 4 years so in theory the current labour government can remain in power for that period of time.
With the majority in the house it is unlikely that a vote of no confidence can be obtained by the opposition parties to force a general election. However if Brown takes over as PM he may wish to call a general election in a couple of years if he feels he will win to give himself a mandate from the country so to speak.
For your information our government including the PM are made up on MPs who continue to serve as MPs as well as running a government department and their cabinet duties.