Such places sell past season stuff and slight factory seconds.
I've not been to Bicester Village in years, it used to be just full of factory outlets, and you could get some good deals. I hear it's become much more mainstream these days though and is more of just a regular shopping village, as there isn't much else in that area anyway.
If you want the latest stuff, where you will be paying full wack, you are best looking in places like House of Fraser (Oxford St one has huge section), Selfridges, John Lewis and there own shop on Bond Street.
If you are buying online, you probably just want to make sure you buy from a reputable retailer, due to the amount of fakes.
I went on a holiday to Mauritius some years ago, and they actually used to make a lot of the shirts there, I'm not sure if they still do. They were stupidly cheap.