Hi, and welcome to the forum! I agree with jpierre that you are going to love this route. As you probably know, you're going to have a tough decision when you get close to Gijon, whether to turn south and connect with the Primitivo in Oviedo, or whether to continue on the Norte, which stays along the coast for several more days of walking and then turns south towards Santiago in Galicia, the town of Ribadeo to be exact. I have walked both of these routes and all I can say is that it's a tough choice but you can't make a wrong choice, because they are both beautiful. What I and others usually suggest is that you wait to see where your "camino family" is going to go. By starting in Irun or earlier, you're going to have a lot of relationships developed and you may not be ready to say goodbye when that point in the Norte comes.
If by any chance you are enamored of pre-romanesque architecture, the decision becomes much easier (I know the odds of that are small, :wink: ). But the church/monastery at Valdedios is incredible, and the church and palace outside Oviedo on mt. Naranco are equally stunning. Other than that, both caminos have all the beautiful scenery, opportunity for solitude and contemplation you could ever want. And both definitely have the Camino "feel" to it, IMO.
As far as travel route, there are so many different ways to get there. I think a lot of west coast people fly into London and then find cheap flights over to Spain. London is well located for both your beginning and ending points, because there are flights to Bilbao and then back to London from Santiago (though they may involve switching airports). I think it's important to get the travel arrangements that work best for both the arrival and the departure, that is, to make sure you can easily get to your departure city from Santiago. Others manage to get "open jaw" tickets -- that is flying into one city and out of another, so there are many many options.
I live in the midwest, and for me it always works better to go in and out of Madrid. When I walked the Norte, I flew into Madrid, hopped on a bus to Bilbao, met my walking partner there (she flew SF to London to Bilbao), we spent a day there enjoying the city, and then the day after that we took the very short bus ride to Irun. Since Bilbao is so early on the route, we didn't think we'd want or need a rest day there so we did our touring while getting over jet lag. It worked very well for us. Best of luck with your planning, and no I don't think there are any places where reservations are essential, especially at the time of year you are going. Buen camino, Laurie