Critique Itinerary

You are probably underestimating some of the travel times--1/2 day to get to Canterbury, look at the cathedral and get back will need a pretty swift trot round what you'll have come a long way to see. Also, driving from Oxfordshire to Keswick via York puts a dog-leg into the journey. Nevertheless, if you don't mind that, this should work out.

Get the Good Pub Guide to find places to eat in the Cotswolds (and elsewhere).

I'd echo Pete's thoughts.
-- I would feel more comfortable with an overnight in York.
Mickleton >> York >> Keswick is about 6 hours driving, leaving little time to actually see anything in York.
-- Canterbury will realistically take up an entire day from London.

Suggestions for your day trips from Mickleton ...
1) Oxford and Blenheim Palace
2) Cotswolds Villages
3) Bath and Wells
4) Stonehenge and Salisbury Cathedral
5) Stratford-upon-Avon and Warwick

Appreciate your thoughts; I've been to Oxford, Blenheim Palace, Stonehenge, Salisbury & Stratford so concentrating on Bath, Chipping Campden, Stow on the Wold etc. & Cardiff.
You're right, we'll spend more time to Canterbury.
As for the drive to Keswick, we really don't have time to spend longer in York (sigh) and don't mind the drive; this will be our longest drive and if we leave really early from Mickleton (after 6 days we can leave early; 7a.m.), we can be there late afternoon even with a stop in York.
Thanks all,
Jean

Make sure you get to the beautiful Hidcote Manor Garden, which will be very close to you at Mickleton.
http://www.nationaltrust.org.uk/hidcote

On the day you drive from Mickleton to Cardiff, I would go via Tewkesbury & take the M50 > A40. (At Raglan. you may wish to loop into Raglan Castle (ruin) for a quick look before taking the A449 towards Newport & then the M4 for final leg into Cardiff). On the return, I would use a completely different route:> leave Cardiff & follow M4 over the Severn bridge & then M5 north east to come off at Junction 9 to head back to Mickleton. (doing it this way avoids the Severn Bridge toll). DO NOT drive through Cheltenham or take the A48 route from Gloucester as these are much slower. My suggested routes will be about 1 hour 30 minutes from Mickleton to Cardiff.

http://cadw.gov.wales/daysout/raglancastle/?lang=en

Also consider a day trip on this route from Mickleton:> Tewkesbury (see Abbey) > Ledbury (ye olde England style small town) > A449 towards Ross & then A40 towards Monmouth - where you turn hard left at traffic lights for A466 going south, > down Wye valley via Redbrook to Tintern Abbey (ruin) > Chepstow Castle & then use A48 via towards Gloucester & back to Mickleton.
http://cadw.gov.wales/daysout/tinternabbey/?lang=en
http://www.castlewales.com/chepstow.html

I think that it is too much to drive from Mickleton to York & then across to Keswick. No dual highway exists between York & Keswick.
http://www.theaa.com/route-planner/index.jsp

Pitlochry is not exactly handy for Balmoral. A mountain road (A939) does go south from Grantown to Deeside (Balmoral is by Crathie) so you could do this route (via Tomintoul) to Pitlochry. However, I would take a look at it on Google Earth before deciding if this is for you. Nice scenery is found west of Pitlochry.

York to Keswick has a mixture of single, dual and three lane roads. I've done the journey in to Keswick in just under 3 hours.
A59 - single
A1 - dual and three lane
A66 - single and dual

For your first part of the journey from Gloucestershire to York I would also allow for at least 3 hours (without breaks).

It sure will be one tiring day you have planned!