Faro to London

I'd do some short tours (even if it's only for a weekend) before you try to do something as long as that. Apart from anything else this will give you the chance to test out your bike and kit.

It would make it easier for people to offer advice if you were a bit more specific about what you want to know and you gave some more information: eg are your riding a road bike? are you planning to camp or stay in hotels/hostels? are you planning on cooking, or eating in restaurants?

As far as the route is concerned you might want to check out the Camino de St Jacques (which has road and off-road variants) which goes (in reverse) Santiago de Compostella -Oviedo-León-Burgos-Pamplona-Roncesvalles-St-Jean-de-Pied-de-Port. Beyond that, I'd be tempted to follow the Atlantic coast and get a ferry from Normandy to the sourth coast of England and avoid Dover. But equally you might want to go up the Loire valley from Nantes to Orléans and then onto either to chartres of Paris. It all depends on your interests and priorities. The best thing would be decide on the places you want to visit and then plot a route between them - I know that sounds like obvious advice but I don't think there's another way.