You are never "stupid" when you admit you dont know anything about a subject, that is the first key to opening the door to learning. Irish is correct, being big and strong is always an advantage and he is also correct, you have to decide which one will be your primary focus, martial arts or gaining strength and size.
You stated you were in the the UK, from my brief trips to London, I was amazed at the amount of different martial arts being taught. A trooper I was with, told me there were over 5 thousand schools in the greater London area. Dont know whether that is accurate, but, I certainly saw about every style known to man being advertised.
IMHO, I would check out two grappling arts (Judo, BJJ) and two striking arts( Boxing, Mauy Thai). This will give you a good introduction for both parallels. If these dont appeal to you, you can try the traditional arts ( Kempo,Tang Soo Do, Shotokan, etc) Welcome and good luck.