Finalizing plans

I would have a 1 night stop-over in York on the way north. http://www.visityork.org

It is possible to fly from Edinburgh to Cardiff (or Bristol) - see www.flybe.com for Cardiff & www.easyjet.com for Bristol. (Easyjet may have a strike coming up).

However, you may prefer to travel by train but the one way fares between Edinburgh & Cardiff/ Cheltenham/Bath/Oxford can be expensive. To reduce the costs, advance book and check split ticket prices. Places where splits have been known to work out cheaper for the overall journey are Preston & Crewe. (For Cardiff, you change at Crewe for an Arriva Trains Wales service via Shrewsbury). The Edinburgh routes are served by www.virgintrains.co.uk
Cross Country trains have direct services between Edinburgh & Cheltenham / Bristol (for Bath) but they use a 1 hour longer route via Newcastle & York.

I mention Cheltenham because that can be used as a base for the Cotswolds - which lie to the east of the town. However, although local buses exist, the villages are best toured by rental car. http://www.cotswolds.info

Also consider a day or two in the English Lake District. Windermere would probably be the easiest place to get to using the trains.
http://www.lakedistrict.gov.uk

Cardiff is worth a visit but her in mind that you would be missing out on the scenic areas of Wales. www.seewales.com run day tours in a mini bus from Cardiff. See also www.visitcardiff.com
http://www.cardiffcastle.com;
http://www.museumwales.ac.uk/stfagans/
http://cadw.gov.wales/daysout/caerphilly-castle/?lang=en;
http://cadw.gov.wales/daysout/castell-coch/?lang=en

www.visitengland.com
www.visitscotland.com
www.visitwales.co.uk
http://visitbath.co.uk

Train times & fares :> www.nationalrail.co.uk (Advance book selected trains for cheapest long distance journeys).
www.traveline.info will give you bus routes.

I'd stay a night in York.

Two days highland tour?, seems a lot, it's very nice, but all looks the same after a time.

Would be pretty cheap to fly from London (Stansted, it's not in London or even close, but you can get the train there) to Edinburgh and work your way down the country, I'm guessing, but it may be cheaper than the train.

We drove from Inverness, along Loch Ness, through the Highlands, past Aberdeen and Dundee to Edinburgh, very nice. Satnav essential.

Marie, definitely stay overnight in York. Regarding suggestions of places to stay, last Fall my husband and I stayed at the Abbey Guest House in York. It is about a 10 minute walk from the train station, right on the river, with a lovely and short walk into the historic area. Their website: www.abbeyghyork.co.uk

In Edinburgh we've stayed at a number of places, but our favorite for convenience to and from the Waverley train station and the Royal Mile is Fraser Suites (where we'll be staying about the same time you'll be in Edinburgh). An added bonus (for me at least) is that they have a laundry room for guests use, which is really handy when on a trip for 3 weeks and trying to pack lightly. Their website: edinburgh.frasershospitality.com

Have you made reservations in London yet?

Also, if you haven't booked your flight yet, I agree with Emma's suggestion to fly into London and out of Edinburgh (or vice versa). We did that last year, into Edinburgh and out of London, and will do so again this year. The Edinburgh airport is a dream compared to Heathrow, which is especially welcomed after an overnight transatlantic flight. Once we landed, the time it took to go through Immigration and Customs and retrieve our luggage was less than 20 minutes. The bus or tram to take you into the city is right outside the airport entrance.

I agree with others about spending a night in York. We too stayed at the Abbey Guest House and liked it very much. It's on the river, just a few blocks from the station and also from the city center.

Your pace is pretty fast by my lights. I'd take seriously the suggestion to fly back from Edinburgh to London. As for Cardiff vs. Bath vs. Oxford, I'd suggest you pick one as a day trip from London (or maybe an overnight if you choose Bath or Cardiff). London offers enough by itself to fill your whole time if you wanted!

Thanks for all the help. Looking into the places to stay in York and Edinburgh. Appreciate all your comments. They've been a big help.