My name is João Ferreira and I must say I admire your sense of adventure planing to drive from the UK to Portugal. Well, in 1995 I did it from Lisbon to Berlin in a time when I only had paper maps instead of GPS and I never took the wrong turn. Actually, as it turned out, I had lots of fun.
But since this is not about me, but rather about you, here it goes:
First tip: drive on the right side of the road and be careful in crossroads (LOL).
Second tip: prepare for a very high fuel price in Portugal.
Third tip: when in Portugal it pays off choosing the motorway instead of country roads; toll prices might appear to be a little bit high, but portuguese motorways are of good quality and you will be saving lots of time and petrol; besides, driving on motorways is a lot safer too.
Fourth tip: use the resting areas and petrol stations to make a pause once in a while; don't rush; Portugal will be waiting for you just the same; driving under stress or fatigue can be very dangerous.
Fifth tip: don't be alarmed by the portuguese way of driving; sadly some of us tend not to stricktly follow traffic rules.
As a first approach I cannot remember anything else. Hope I am being useful though. Don't know your final destination in Portugal, but if you are coming near the Lisbon area, I shall be delighted to be at your service within my reach. By the way, Lisbon is wonderful during summertime, because everybody is out in the Algarve and you can easily circulate in the city with low traffic.