Thursday afternoons (or Fridays) are one of the better days, as most places have received any new stock they're due that week, and had a chance to unpack them and put them out, but the weekend hordes haven't yet had an opportunity to descend like locusts upon them. Point of fact, locusts are actually tidier.
Some outlets get massive tranches of new stock irregularly, others get a gradual drip-feed on a more regular basis, you just have to get to know what they do. RL tends to be one of the ones who get big batches towards the end of their regular stores' sale season. They often get the previous years stock for the next season around then, too. The same principles apply to learning the timing of their major price cuts. RL, for example, tends to cycle through 2-3 different price cuts over any given season, so if you time it just right you can still get quite a lot of choice, plus a lower than expected price.
(PS. I don't visit Bicester Village anywhere nearly as regularly as I used to, so don't know the details of any of the shops' current routines, so there's no point in PM'ing me to ask.)