Hmmm Ok makes sense. I've also noticed that any time I've gone before, no matter what I said for my duration of stay wether it be 1 week, 2 weeks, or 3 weeks, they stamp my passport for 6 months. So let's say I told them I was only staying for 14 days but later decided I wanted to stay the full 6 months. Could I do that? Or is that frowned upon...
In theory it can be possible to do that, but it would have to be a genuine, last-minute change of mind once you were in the UK and it can be risky.
If you entered the UK saying that you were only going to stay for 2 weeks, but you actually intended on changing your ticket as soon as you were let in, that is considered deception and if you were caught in the lie, you could be refused entry and banned from entering the UK for 10 years.
However, if you genuinely intended to stay for 2 weeks, but then something came up just before you were due to leave and you had to extend your trip unexpectedly, that isn’t so much of an issue.
BUT, the visitor visa rules are changing in the next few weeks and they will be becoming much stricter with visitor visas.
As you’ve noticed, you have previously been stamped in for 6 months even if only staying for a couple of weeks – as this is the standard visitor visa stamp. However, from March 2015, the Immigration Officers will have the power to stamp you in for only the length of your trip – so if you come for 2 weeks, you may only be stamped in for 2 weeks and will have to leave within that time.
Also, I believe they may start looking more closely at previous visits and if they see you didn’t leave when you said you would when you entered, that could count against you in the future.