I think that I may have to disagree - sorry - with ryan_and_gill about Bicester outlet mall. The place is heaving most of the time because overseas visitors from China, India, and the Middle East flood the place by the mega coach load, and Chiltern Railways has a stop very near the shopping mall on its route from London Marylebone.
I have nothing personally against the tide of visitors there - bring your money and leave as much of it as you can, is what I say - but it is the sheer weight of numbers that overwhelms the place. It isn't many times in my life that I have seen tour groups following the little umbrella - at a shopping mall.
Also understand that the mall is not covered so if the weather doesn't cooperate you may get wet going between shops.
It pretends to be English, but this is a facade to cater to hordes. Expect long queues even in the fast food places like Pret a Manger and Itsu, and long waits for sit down table service places.
So if it is so 'orrible, Nigel, why did you go there? Easy answer - I used to own a business a couple of miles away so went there earlier in its life when it was less crowded and I could get a crêpe without waiting or have a nice Busaba Eathai lunch, again, just wandering in. Now I can't find a parking place without going around their wacky one-way car parks for more time than it is worth, and the queues are ridiculous. Good people watching though, if you do get parked.
I propose a route only about half an hour longer than the direct route (plus wow moment photo stops and walkabouts) -
From Avebury head towards a direct line to Bletchley but turn left before Swindon (unless you want to experience the Magic Roundabout - a gyratory system of FIVE interlocked roundabouts (think of how the gears in a car gearbox interlink) forming effectively one giant roundabout, and you can go both ways around!, and I actually think that everybody who want to drive in England should have it on their list, but Swindon, other than the excellent Steam! museum of the Great Western Railway and adjacent outlet mall is a dump (sorry, Fidelma) ) and go towards Royal Wootton Bassett (recently got the Royal in its name because every time a soldier was returned in a box from Iraq, Libya or Afghanistan via the airbase (also used by Americans) nearby, the entire town would turn out and line the route of the cortege, scattering roses and flowers in the path - the government and people of the country were so moved by this informal recognition of the brave soldiers (and other forces) that the Queen granted the title in 2011. Neat architecture.
From there follow signs to Cirencester, a typical Cotswold large town with a Roman villa nearby to visit, and plenty of refreshment if required. From Cirencester follow the Fosse Way north, signposted to Stow on the Wold. A Roman Road, the Fosse Way will take you through the heart of the northern Cotswolds with typical Cotswold villages to both sides and in some cases right on the road. The name Fosse Way is ancient, the modern name for that portion of it is the A429. Pretty straight, as with many Roman roads, quite hilly, not the fastest road in the world but direct. Various places to stop and gawk such as Bibury (Arlington Row, google it)(off the Fosse Way via the B4425, signposted Arlington and Bibury, when returning to the Fosse Way take the lane along the route of the extremely scenic Coln River (just a big stream really) past Ablington, Winson, Coln Rogers and Coln St Dennis - signposted Fossebridge). Enjoy going slowly along the lane, enjoy slowing down at every crossroads to read the finger signs. Because of the way finger signs are made you have to slow down or you can't read them.