Upvote:2
Being allowed to stay within the country after the expiry of the visa is extremely uncommon and probably the US are the only country that allows this (under certain circumstances and for certain visa types, such as student visa).
If your visa is valid until the 25th of February 2015, then this is the last day that you may be in the country, unless (1) you have another UK visa for after that day, or (2) your cleared passport control on an international airport at the 25th while your flight is (early) on the 26th. In both cases, there may be some non-trivial process that your would need to follow, though.
Note that when clearing immigration on the 18th, the immigration officer may want to see a proof of booked onward travel for leaving the country before or on the 25th. This is actually quite likely to happen if your enter the UK a few days before your visa expires. So you are strongly advised to be prepared for this case.