I have been driving down for the last 5 years and did it 3 times last year as I was bringing goods to furnish a flat I bought. Like you I am the only driver.
I don't like paying toll charges so my advice relates to non toll routes.
The least driving route is getting a Britany Ferry from Portsmouth/Plymouth to Santander/Bilbao and you can drive down from there to Alicante in a day easily. The only downside is that the ferry is quite expensive unless you manage to find a special offer. I have done this route a couple of times.
My usual route is to get a Dover/Calais ferry or Eurotunnel and dive through France roughly via Rouen, Tours, Poiters, Bordeaux and Pau. The first time I drove down and not being sure how long it would take me I made overnight stops at Rouen, Bordeux and Zaragoza in Spain. I found out I was arriving at my next stopover by late afternoon so now I only have two stopovers. Going down I usually stop the first night somewhere between Evereux and Blois and the second night at Pau Lescar. For the return journey I stop at Pau again but stop further North in France, somewhere between Blois and Rouen, depending on how much time I want to spend shopping in Calais. I usually stop in B & B Hotels or Etap Hotels which are now rebranded Ibis Budget. They cost 40-50 euros for a double bedroom and buffet breakfast extra. They can be booked online and if you are delayed you can still get in as they have automated machines outside to give you your room number and entry code to get in. Also, if you are delayed and want to alter your plans you can cancel it for free if you notify them before 18.00
Although I don't go on the peage I should think about 60-70% of the route in France is on dual/three lane carriageways and 80% in Spain. Watch out for speed cameras in France, they are not like ours but grey square boxes, some about 1m high and others 2m high. Up until last year they had to by law put a warning sign before them, but not any more. They have left these notices up but on the newer ones if you see a digital readout sign with your speed expect a speed camera within the next 0 - 5 kms.
I hope this is helpful, if I haven't covered all you want to know, let me know and if I can answer it I will and if I can't I am sure somebody else on here will.
I have a friend who drives down with only one stopover at Bordeaux, but it is too much for me.
I have just thought, if you want to spend a few Euros on tolls, I would consider the peage from Boulonge to Abbevillle, which I think is about 15-20 euros, as the non peage route is a bit slow and twisty and every time I say I will try it but haven't put my hand in my pocket yet.