It's a possibility.
Basically, the rule of thumb is that no genuine visitor should spend more than 6 months in any rolling 12-month period in the UK. If they try to spend more time than this in the UK, it can look like they are trying to use the visitor visa to live in the UK, which is illegal.
You wouldn't definitely be refused entry if you turned up at the airport, but it's quite likely that you could be. It's up to the immigration officer though as to whether they allow you into the UK or not, depending on whether they believe you are a genuine visitor or not.
Same goes if you apply in advance for a visitor visa - in both cases you would need a very good reason why you are returning to the UK so soon and lots of evidence that you have ties to the US that you have to return to which will prevent you from staying in the UK.
For example, you would need to show:
- that you have a job to return to in the US (a letter from your employer stating that you have been given time off for your visit and what date you will be returning to work)
- that you are maintaining a home in the US that you must return to (evidence of homeownership or a valid rental agreement)
- that you have other ties to the US that will prevent you from trying to stay in the UK and that you must get back to (important appointments, interviews, social occasions, family you need to look after, etc.)
You would also need to show enough money in your account to support yourself for the entire visit without working.
Basically they need to see that you have a life in the US that you must return to, and that you have no reason to try to stay or live in the UK. The longer you try to stay in the UK and the less ties you have to the US, the more likely you will either be refused entry or refused a visitor visa is you apply in advance.
Personally, as you have already spent the allowed amount of time in the UK in the last 12 months, I would not attempt to enter the UK again until at least February 2015.... So that you will have been out of the UK for 6 months and won't end up spending more than 6 months in 12 months in the UK.
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