Traffic round Oxford on a Friday Evening

[omniflow]

I'm looking for some tips on Friday evening traffic around Oxford on a Friday evening. I've got to get from Swindon to Great Missenden, leaving Swindon at 4:45pm with no flexibility on the departure time. I've tried using M4 / Marlow Bypass, and that's a definite no-no at that time on a Friday, so I'm thinking of going via Oxford - even though the road from Swindon to Oxford is really slow for the first 15 miles, I figure it might be quicker. I have 2 choices, direct to Oxford from Swindon, or to Burford and then A40 to Oxford (further, but potentially quicker). In both cases I would take the bypass, and then head to Thame at the turn-off for the services.

Can anyone let me know what kind of traffic levels I'm likely to encounter. Thanks.

[fatboy b]

Best to avoid in any rush-hour tbh. My O/H works there, and can take 1.5 hours to do the 36 miles home. The first hour is about 5 miles on a bad day.