Firstly the -300 can be tight on gas if the flight home is into stronger headwinds than normal.
TCX have a policy of carrying the right amount of fuel. This has been emphasised for years. What they don't want is an extra couple of tonnes for the wife and kids, mainly because if you do that on a regular basis you probably won't carry the extra 4 tonnes when you need it.
Hours only become a real issue when you divert due to the fact that you generally lose around 45 minutes for the extra sector. So although you can just about get back from HRG without the level 2 extension, once you divert have time on the ground and lose 45 minutes then things become a bit tight.
As for runway changes.........I don't know. They generally only happen at LGW due to a change in wind direction!