I have done it from Inverness: Rode to Dover, ferry to Calais, did Spain in a day and took the Algeciras to Ceuta ferry. Similar route to your plan as far down as Merzouga with all the usual bits in-between. For the return i rode from Lille to Dunkerque, ferry to dover and back to Inverness in a day.
It all depends on how much time you have, I was forced to do Spain in a day as I had planned to take a leisurely route through the Pyrenees via the tunnel, but halfway up the road was closed due to snow which only got worse, so had to turn round and ride around the Pyrenees losing almost a day from my schedule, so the race across Spain was simply to catch up on my itinerary.
This was back in 2004 and no doubt things have changed but the border crossing at Ceuta was a nightmare with corrupt officials, they forced you to use a fixer who of course charged you for the process, your documents were processed based on how much cash was in them, the fixer would set the rate depending on how busy they were, so for example 10 euro in your passport and it would go to the bottom of the pile, 50 would put it to the top, that could mean the difference between a one or five hour wait. After that and the import paperwork for the bike (make sure you have your V5) you were then made to buy insurance (that you don't need) as you no doubt have your own and finally the last hurdle was the policeman who opened the barrier to let you in, he wouldn't unless you gave him something.
Be interested to know if its still the same?
Everywhere I went I was constantly pestered by scammers and robbers, threats to your bike and luggage when parked unless you paid someone to mind them, if you take a picture in public anyone who thinks they may have been in it will demand money. I remember sat on top of a sand dune in Merzouga to watch the sun go down, two figures appeared in the distance and about 40 mins later they struggled up the dune, they then put their hands out demanding money, I ask why and was told it was because they had made the effort to come and speak to me and I should be grateful for that.
In Marrakesh they tried to snatch anything off you as they walk past as they can then disappear into the crowd, so keep a grip of bags etc.
I got chased in the Rif mountains by drug dealers who try and sell you hash wether you want it or not.
Don't let the above put you off as things may have changed since then and no doubt someone with recent experience will be along and after all its all part of the adventure.